copper bulbs for thermostat dry well

Anyone know where I could be able to get some dry wells around 3' that I could use to read the bottom temp of my propane tank thermal storage? The only ones I can find are the Honeywell 3". Or if anyone knows how to make one this size.... I am having trouble finding a 3/4" male thread with a 1/2" sweat fitting on it that would allow me to screw into the 3/4" ports I have on the top of my tank.

Can you use a strap on aquastat or aquastat with a surface mount sensor - then you won't need a well? Also, you should be able to bush the 3/4 down to 1/2 then find a 1/2 thread/sweat adaptor?

Triangle tube uses long dry wells on there water heaters. I believe they are 3/4" and various lengths depending on the size of the tank.

Perhaps you could get a standard 4" dry-well (or 6" ?) and weld / braze a brass rod onto the end of it to extend the sensing surface.