A portable hottub

This is a nice CNC based build.  The author at the end says you could get away with 5/8s, maybe 1/2 inch plywood instead.  I personally like the water heater he used that is propane based and only $100.  I think I could cheap out on the water pump and maybe use a cheap pool filter instead of the one he does.  Of course to make this you need a full sized CNC machine.

http://www.instructables.com/id/Portable-Hot-Tub/