You sure can make something like this for a lot less the $650.00. Frame it up with 2x2's and sheet it with 1 inch 4x8 sheets of foil face foam insulation. You could paint the foam and it would make it very light! I just sketched up a plan 3 feet wide and 2 feet deep my estimate would be less then $80.00 and if you wanted to go bigger maybe $100.00 This would be fun to build!
Here is one I though would be nice also. Google: Silver Shanty. I know they use it for a spearing shanty but you can make into what you want.
Always dreaming!
Beeler