I drill though the tube so each insert is welded around each end. It's solid!

25mm Box, 2mm Chequer Floor, Bolted throught with noo crush.

TT

i cut some 25mm box today and put the rivnuts in
it does look quite solid
but im going to do 2 tubes fitted with crush tubes and then ill decide

I used m8 rivnuts in 25mm box, worked great and very neat. I use the theory it is the 4 point harness that takes the load in an accident, not the seat.