In Dale Kirn's article for the Lil Pronto (American Modeler, Sep/Oct 64) he shows a hard tank and a soft balloon tank. The balloon tank (attached, might need to enlarge to see details) is not inflated like a bladder, just used as a soft fuel container. He said to squeeze all the air out then fill without inflating so that when the filler is remove no fuel comes out. Seems this then just collapse as the fuel is drawn out by the engine. I think one could do this type of non-pressure soft container by using fingers from a large vinyl or other type of glove. This way you could see fuel/air in the fuel sac and get out the air. I think this would fit in almost any shape holder like oval shape, smooth wedge or round and sit on the side of the fuse nicely. I intend to try this on the test block and see how the whole thing works. If anyone has used this approach please post your experience with it.
Did a little search and the RC pylon and combat guys have been using this type tank for quite a while it is called a Bubbless fuel tank system.
Best, DennisT