I use some of the threaded inserts for wood. They screw into a hole then you can screw a bolt or screw into the insert. They are sized according the desired screw or bolt size. 4-40 6-32 1-4” etc. A search shows McMaster-Carr sells them made for plastic. They may have some you can use.
Du-Bro sells brass type made for wood. Not sure if they sell for plastic. You might check with them.
