Hi Max-
I wet form. Leave your back side a little longer than you think you will need, it will shrink a tad after drying. I usually wrap the piece with another piece of leather and rubber band it to the flask. Set the front side down on a shooters bag (don't tell hubby) then place a plastic coffee can filled with marbles on the concave part of the back and let dry. This will get you your shape. Trim the back edges to fit after drying. I haven't glued any to the flask itself, but I do use a piece of double stick tape on the seam (after lacing or sewing) to help it hold it's shape to the flask.
I have also made a leather boot for the bottom and riveted to the main body that had straps on the top to make it completely removable. I do this with the 64 oz flasks.
Flasks are fun- you can also play "hide the seam".
Crystal