The tube that holds the batteries is ½ inch PVC pipe.  You can use water pipe from the plumbing section of the hardware store or what I prefer is PVC conduit.  Menards sell PVC conduit in 4 ft sections and the PVC fitting for water pipe fit it. Cut a piece 17 inches long.  It will be cut a little shorter later when the exact length can be determined.
You will need two ½ inch PVC caps. Drill 1/8 inch holes near the center of the caps.
Two non-corrosive (brass or stainless steel) screws with 6-32 thread and five 6-32 nuts out of the same material. The best length would be ¾ inch to 1 inch and they need threads the full length. They can be any head shape but I prefer round head. Put a nut on one screw and tighten it against the head of the screw. Put the screw through the hole in one of the PVC cap with the head of the screw inside the cap. Add 2 ring lugs with the flat side of the lug toward the PVC cap and add one nut.   (Ring lug picture.   (You need to Login or Register ) Position the 2 ring lugs so they point 180° from each other and tighten the nut.  It will be difficult to tighten this nut in the future and it should never need to be loosened so make sure it is tight. Sand any burs off of the ends of the 17 inch piece of PVC pipe and glue this cap to one end. Mark this end cap as the negative end of the holder (NEG or –(black wire)). Set the glued pieces aside to dry.  Place a screw through the hole in the other PVC cap add 2 ring lugs and a nut. Position the 2 ring lugs so they point 180° from each other and tighten the nut. This nut should never need to be loosened.  Mark this cap as the positive terminal (POS or +(Red wire)). After the glue has had time to set on the pipe push the POS cap on the other end of the pipe firmly but so you can still remove it.  With a led pencil mark where the edge of POS cap is on the PVC pipe. Put 9 AA batteries flat end first into the pipe.  This is where you may need 3 or 4 hands.  Place the POS cap over the 9th battery and try to hold it straight.  It most likely will not reach the PVC tube.  Measure the distance from the edge of the POS cap to the led pencil mark on the PVC pipe. Take this measurement and subtract 2 inches from it and shorten the PVC pipe by this amount. With all the batteries out of the tube sand the POS end of the tube until the POS cap will slip on and off very easily. Put the POS cap on the PVC pipe with only the force you can apply with one finger and use the led pencil to mark this location on the PVC pipe.  Put 8 AA batteries flat end first into the pipe and add the POS cap.  The edge of the POS cap should be near (1/16 to3/16 inch) the pencil mark but not at the same location as without batteries. If it is in the same location as without batteries shorten the pipe 1/8 inch and re sand and re mark and re check. With 2 pieces of 1/8 inch shock cord about 13 inches each tie a over hand knot in each end of both pieces.  To tie the shock cord to the ring lugs use small nylon braid or heavy fishing line. Take about 7 inch of line and put it through the part of the lug where a wire would go and tie the ends together.  Use a larks head to attach the line behind one of the overhand knots in the shock cord. Connect the other end of the shock cord to one of the ring lugs on the other end of the holder and add the other shock cord between the other two ring lugs. Put the cap on the end of the pipe with less than 8 batteries in it and mark where the edge of the POS cap is on the PVC tube. Put 8 batteries in the tube and add the cap.  Note where the edge of the POS cape is it should be just short of going on as far as it did without batteries in the holder.  One more test pull the cap off about 1/8 inch and let go of it.  It should snap back to rest on the batteries if not sand to make a looser fit or tighten the shock cord.  Add a 6-32 nut to each screw to use to fasten your detector wires to the battery holder.  The holder is done but it needs to be mounted so the POS cap is free to slide on the pipe.  If it is going to be mounted with bungee cord so it can easily be removed all that is needed is a spacer to keep the POS cap from binding.  The lumber section of the hardware store usually has some wood about ¼ X 1.5 X 3 ft. cut 2 pieces about 1.5 inshes long and glue them to the side of the PVC pipe with GOOP.  What I prefer is mounting the battery holder to the side of the canoe under the gunnel.  If you don’t want to stand the canoe on end to get the batteries out of the holed the holder needs to be removable.  To make the holder removable I use TOOL HOLDERS like   (You need to Login or Register or TUBING HANGERS like   (You need to Login or Register .   The TOOL HOLDERS are easier to mount and hold tighter but are harder to get.  Both need the mounting screw put all the way through the wood and sticking out the back side then removed and shortened and reinstalled.   
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To use the TUBING HANGER pull the nail out and throw it away.  Drill a hole 90° to the nail hole and the pipe direction thru the plastic for a wood screw.  Use a larger drill bit to make a relief for the head of the screw.  I used a ¾ inch flat head wood screw which stuck out the back of the wood less than ¼ inch.  I used side cutters to cut it off and a file to reduce the bur before re installing.  To mount to the canoe I placed the clips on the tube.  Tipped the canoe up on its side so the place where I wanted to mount the holder was down.  Smeared a thin layer of GOOP on the bottom side of the wood and pressed it into place and let dry. You are ready to add batteries and connect the detector.