I just came across this problem with the birthday phone I recently bought. It had a plastic transmitter cap on a 1954 G1 bakelite handset.
The cap came off OK when I was cleaning and polishing the handset, but when it came time to reassemble things, it didn't want to go back on. This is the first time I've ever encountered this problem. I found that even my funkiest of parts phones with G3 handset wouldn't readily accept this cap.
I traded caps with another phone I'm not all that attached to and let it be, partially screwed onto that phone's handset. Later I checked another small stash of phones and parts and found a cap in there. Although made of plastic, it hadn't shrunk enough to be a problem and didn't offer too much resistance when installed. I have since separated out the shrunken cap since it's practically worthless on anything but an accepting G3.