You have to rip the liner out. It's George glues the liner in at the bottom so you need to forcibly tear it out.
Then you can rivet the strap on. You will need a thick iron bar to use to set the rivets. You will need to anchor this iron bar in such a way that it sticks out from the table so that you can put the case onto it and hammer the rivets without the bar moving.
Something like this is what we built based on a similar one I saw at another case maker's shop.
Once the rivets are set you can glue the liner back in using Super 77 spray glue. Let it get tacky before you put the liner in. You might need to repair the bottom of the liner first as it may be torn due to the removal. You DO NOT need to reglue it to the bottom.
DO NOT ATTEMPT TO TAKE THE BOTTOM OFF. REPEAT DO NOT ATTEMPT TO TAKE THE BOTTOM OFF.
It's George use pvc glue to bond the parts which effectively welds the bottom cap to the tube. Trying to force it off will generally break the end cap or break the end of the tube or both.
Everything else you or the shop needs to do should be self-explanatory.