Always interesting to hear the way others handle practical, everyday, canoe issues.

Once I start paddling for the day, the map case stays INSIDE the canoe.  Like you, I mount to the thwart, but use either velcro straps or dealee-bobs.  I anchor a corner of my map case to my foot rail when portaging so it doesn't fall in front of my face when the canoe is upside down.  I'll do both corners if there are other floppy items attached to the thwart (e.g., sunglasses on a neck strap).

As far as the GPS and depth finder ... they are both attached to the foot rail, one just a little on each side of the center line.  It is not uncommon for both to stay in the boat overnight.  In this positon, they are out of the way when portaging (can't snag them) and they are below the gunwale line when the canoe is parked (unside down) overnight.  (I used a bicycle mount for the GPS and a electrical conduit clamp for the depth finder attachment.  The way I attached the conduit clamp to the depth finder allows the face to rotate fore and aft as well as port/starboard so I don't have to fight sun reflection.)

dd