I found this on the forum :
Securing lid
Essentially, this is both option applied by other users. The option with drill and tap is on aluminium casing.
I agree that thread insert in acrylic may be hard on 1/4 inch wall. I already tried this in another application and it crack and was not stong . Another way would be to glue some more acrylic on the side of the tube and install a thread insert there or drill a big enough hole to install nut and bolt justr like a plumbing flange. It would also require a bigger acrylic end cap but Blue robotic could probably easily take the 3" drilling template and apply it to a 4 inch end cap leaving enough peripherical material to fix it while keeping the hability to screw the cap to the flange.
I will report my tests results for futur users.