Im not much of a builder. But I did think of a way I think may work.

5 large pieces of rectangular wood
Make a box with the wood that has a bottom
Nail it all together
Put silicone or whatever on all the edges to seal it in.
Put something for steps inside.
And use a piece of big meta or steel as a cover.

Ill just use hose watr to fill it and heat up rocks and put them in to heat it.

Will this idea work? If not, what can I do to make it better. What kind of wood should I use. Help.