JM's advice is as good as you'll get for crust. Crust stays on the hook a surprisingly long time so long as its not nobbled by rudd etc. Best to use a loaf that's a couple of days old so that the crust has got a bit rubbery.
Large lumps of freelined flake will also float so long as you do not squeeze it on too firmly.
A month or so ago I carried out some tests in an aquarium with flake and a couple of hooks sizes to mimic float or lead fishing with a bottom bait. A 10p sized disc of bread pinched on the shank of a num 6 hook stays on for about 8 minutes. It swells up quickly upon immersion and then starts to "unhinge" from around the shank, eventually falling away from the hook. This process would be much faster in running water. In still water the flake really pops up, to the extent that it lies adjacent to the line going down to the bottom shot (so line is shaped like a "U"). A recipe for a lot of false knocks unless you fish overdepth and have the line angling down from the float. Or you could use two spaced shot lying on the bottom.
Small pieces of flake on say a size 14, especially punched bread, stay on for almost twice the time described above.
I also tried two penny sized pieces of flake hooked back to back. I quite often leger for chub in this way. I was stunned to find that it took two SSG to keep it on the bottom. Remember this is all in a fish tank. How does it behave in a fast current or even a slowly paced river?
Time messing around like this is time well spent. No need for a fish tank, a plastic bucket will do. I needed photos so it had to be the aquarium for me.