I've never had issues with thermals in my scopes. My first builds were in sonotubes, and everything was fine. When I refurbished all of them and put them in aluminum tube, I lined the tubes with cork, and didn't have any thermals.
My 12" Hardin has thermal issues. When I first used it, the out-of-focus stars looked like triangles, and the stars were not tiny points. The mirror was pinched by the 3 rubber spacers that held the mirror in place. I reworked those spacers and it helped a lot, but the stars still didn't look right. I took the cell apart and the 9-point flotation system was not working as intended, so I rebuilt the 3 small triangles, and reassembled everything.
I also ended up reworking the secondary holder to make it easier to align. At that point, with everything aligned, only the faint stars were tiny points, the brighter stars were "bloated", but they were round. IIRC, this could maybe be two things, thermals, or bad optics.
So, now I'm going to try and eliminate the thermals, by using a fan to blow on the mirror from the back.
Here is the fan support made from a piece of flashing, that will be held on with small pieces of Velcro. The holes are for the adjustment bolts.
The fan will be powered by a small 12v battery pack.
If this doesn't work, I will then line the steel tube with a thin layer of cork, and if that doesn't work . . .