tube ends
 

What do most use to close off the ends of the chassis tubes; square plastic bungs or metal sections welded over the ends

Welded mine.
I chopped a spare bit of 19x19mm tube into 19mm lengths, then chopped them in half to get U-shapes.
They were a nice snug fit into the end of the 25mm chassis tubes and stayed in place while I welded them.

Great idea thanks will try that one myself