This is a repost since my original message was in the wrong forum.

I have finally gotten the cabinet for my shell built and have the shell in the cabinet. Only problem, the bottom of the shell is about 5.5" from the ground. Since we don't ever plan to move the tub my thought was to use some 4x4 posts (since I already have them) or something of that nature and lay out under the footwell and then fill the remaining 1.5" or so of space with the spray foam that they use to coat the shells with. Do you think this would be strong enough.

no, foam will not support the weight - the weight of a full spa needs proper support to ensure you do not end up with cracks

Stick additional 2x4's flat on top of the 4x4s...

Me? I'd probably add 2x4's then some good exterior plywood filler to bridge that gap for a sure fire support. If it pushes the shell up a bit, then that's ok as the water weight will probably push it back down in the end.

It's not important that it 'looks cool' but that it really WORKS. I've seen really ratty looking supports that were an eyesore under a deck from blocks to sand to left over construction material; crappy looking but as long as it's solid, then you'll be ok.