Snorkel hot tub in greenhouse for a possible conversion to a RMH?

I buried this snorkel 1/2 way down in a 22' dome greenhouse. Its at the low point and all the cold air goes down there. Usually its empty, and so far the "ambiance" in the greenhouse hasnt been up to par for a good hot tub dip. Plus the chimney leaked and I wasnt around much in the winter when it would have been useful. I did this back in 2008, and probably didnt know about RMHs until 4 months ago?

So Im thinking that maybe a barrel would replace that junky rusty (very humid in there and it would just rust within months) chimney and then we could wrap the circumference with the exit pipe? I have the Ianto book but no experience. I am surely cool with waiting for late spring. As far as the upper wall above the hot tub, its like a reflective soft 1/2'' padding. It would melt if the heat was close enough. If we used a small one and then put more protection above just in case?

Or I was thinking I could take the whole thing out and put in a real RMH and have that like a lower sitting hangout spot with the trees starting 4' above. Ideally I think a circular stone/cement tub with a waterproof RMH built in would be so ideal...with a big cob mass against that back wall?

I was envisioning running the exhaust pipe around the front and then back again and covering it all with pebbles up to the first metal ring so our feet would be level with the water as we walk up, with the pipe underneath us giving off heat - YES the pebbles are porous enough that the cold air can still sink into the low spot in the winter. At -25F outside the lowest ive seen in there is +15F. I have two persimmons (chocolate and hachiya, a date plum, goumis (2 and putting more in for sure now) comfrey, fuzzy kiwi, grapes, and some annuals across the inner front in warm months. Im not there in the winter and no one waters in until March when the green starts showing up (about 2 months before outside)...I envision the 42' greenhouses with swimming ponds and cottages inside.

I think the RMH is just perfect for these - and connecting it to the snorkel which I already have (cool concept but losing so much heat still they way they designed it...and it was always envisioned for outside use - until the RMH?

http://snorkel.com/hot-tub-info/snorkel-and-scuba-stoves.php this is the design of the stove - its an aluminum stove to prevent rust - aluminum tea water isnt very cool I just realized it was aluminum by looking on their site.