I agree with Tom I haven't seen the PVC covers sold separately. I never understood why all metal ones don't come with covers although some manufacturers are selling now them complete with the cover.You will need to purchase the conduit body and use the cover if it’s PVC. Malleable covers are available separately.
Let us assume the metal cover fits the footprint of the PVC, how are you bonding that cover to the EGC?Well ,can I put a metal cover on the plastic conduit ? .I've installed 2 boxes I have already glued an wired inside ..thinking hey,I can get a replacement cover..an can't find it alone .which is really crazy when metal ones are easily replaceable ..