FYI, the MG Owners' Club Catalogue (
online here) includes a "Door Seal Retainer MGB" which looks to be the same sort of thing. (The pic in the online catalogue is not as good as the one on the paper copy.)
If anyone lives or works near MGOC HQ (Swavesey, north of Cambridge) perhaps they could pop in, take a look & report back as to whether it would be suitable or could be adapted.
Regards,
Kearton