How about using "fuzzy foam" as the carapace material? Light, slightly buoyant, could be used with no epoxy.
I tied a variation of one of your crab apples last week; this one stands up in the defensive posture, doing so by leaning on a rattle rather than using monofilament to "make a stand" (as some other patterns do). I'll get a couple of pics of it tonight. This fly was designed for redfish, snook, tarpon, and stripers, more than it was for bones or permit. This is the first crab fly I've ever tied that I can truly be proud of. I would imagine that the noise created by "spalsh down" of this fly would spook some of the more wary flats fish... I'd rather hook a jack than a bonefish, anyway.