My two Orion Dobs have a square boilerplate on the inside that allows me to crank down on the viewfinder mount bolts.
My Zhumell doesn't have one and the viewfinder is wiggling all around. I can't tighten it too much or it bends the steel tube.
The Zhumell is a 12" and the tube is 14" in diameter.
Attached is a photo of the one from an Orion 10" Dob.
I suspect it should be 4"x4" steel and 1/16 inch think. I don't have any metal-working tools. Is there a source to buy one of these?