Looking for 3-4 person outdoor sauna. Need help with roof, heating type

I like:

• The style of this one
• The fact that it has two levels plus a floor to sit on for people who want different temperature/steam levels
• I think the rectangle shape is actually a better use of space than a barrel shape

Some constraints:

• It needs to go outdoors, the one pictured has glass which seems like it wouldn't insulate well outside
• My yard will get down to 0-15f or -17C to -10C when I want to use it
• Dry but snowy climate, Western Colorado
• Backyard is quite small, using space efficiently is important

Big issue:

• My panel only has 100 amps and only a 110v exterior outlet. It'd be horribly expensive to upgrade the panel, I already looked into it. And expensive to install an exterior 220/240v outlet.
• Propane heating element seems best for me. I'm not into wood.

Summary:

• Need a 3-4 person outdoor sauna, preferably propane heating element. Any suggestions?