I just use our "tubing" tubes.
This year I might try to buy some of that foam they use in docks. I've heard that works really well also. Something like this...
With these (Dow buoyancy billets are light weight, quick to install and won't lose their buoyancy, even if punctured)
For removing the lift later in the season, I built this homemade jack for lifting the frame up so I can put the float under the cross members and get the wheels on without holding my breath under water. It works pretty well. You can also buy these online.