I remembered you talking about flange warpage earlier and I could only find 3/8 inch at the most. I found the thinner 1/4 inch all day long but knew they were junk.
The output flange is also 3/8 in thick on the ones I am getting. I know that it's not as big of a deal at that end but it does help.
I got them from Stans Hedders in Auburn Wa, Custom made for $800.00
The tubes are 14 gauge steel.