This is a rough drawing to show my idea. It consists of a machine that would be lowered over a semi straight area of the 21" diameter pipe. A hinged clamp would then be rotated around and bolts would be tightened.
A saw then pivots around and cuts a clean surface for a cap to seal against.
The rubber faced cap is then installed with studs and nuts.
There are things that can be done to help with the assembly that I did not include in the drawing. Also for simplicity I did not completely draw the band saw.
I own an Aerospace machine shop near Dallas and I could have one of these machines ready to go in less than two weeks.
This idea is simple and can be installed with rovs.