Quentin
If you and your detector can do it on 2 ryobi batteries that new are listed as 1666 mAH  then 9 Lithium batteries at 4.7 OZ + watertight case with mount at 4.6 OZ  will last you through 1 and ½ trips.  9 Lithium batteries cost about $16.65.  You have a light weight low power consumption detector (I am jealous) but your batteries make up for it.  What does your battery box and tie downs weigh?
For you
9 Lithium AA + Case +case mount ______________9.3 OZ  With power left over/trip
2 Ryobi ________16.88 OZ + pelican box/trip