Fat floats, but is gross. You could form it into a disk with a "lump" (like a diaphragm)(you know the thingy for women lol) so there would be a pocket of air it would float on top maybe if you made them thin enough. Or if you made them "thick/sticky" enough....You could put the meat in a smallish ice cube tray and use a basketball pump with a needle to put air into the meat......Never tried either just brainstorming for you lol good luck! Both would have to be frozen to serve.(I never have a prob with frozen it lasts a lil longer and smaller fish have a chance)
Im assuming your already using gelatine in your mix as a binder, so you could simply add some floating fish pellets to the food mix during preparation to provide slightly positive buoyancy. You could also bind it to polystyrene or anything that floats too but the fish might eat some by mistake.
Alternatively, you could make a floating feed ring with onion skin netting and let the fish eat through the net? Or let it sink and make sure you have some scavengers.