It would take a heluva lot of cans to get there with can-foam.
A 12 board foot FrothPak (or two or three) would have enough do something useful though. An inch of closed cell foam sprayed evenly over a fiberglass monocoque adds some structural rigidity, and 2" adds even more. To avoid stressing the tub you want to spray it in lifts of an inch or less. Too much expansion between the bottom of the tub and the subfloor could warp it or make it lumpy- the idea is to stiffen up the tub (and insulate it, which keeps the tub water warmer longer), not to support it on thin or uneven foam fulcrum point. The big box store home centers carry FrothPak kits- its' a 2- part 1.75lb density polyurethane, comes with a spraying gun and hoses,etc. It'll cover 12 square feet at 1" nominal thickness, and you can spray it from a couple feet back if you have to.