Classic Roadsters offered an adjustable seat frame for the VW chassis. It is intended for use with Classic Roadsters bench seat. But I think it could be modified for use with a bucket.
I bought the parts but never used them and would be glad to sell them. Contact me off line at email@example.com if you want to discuss this further.
Former Owner of a 1981 Classic Roadsters Ltd. Duchess (VW)
2005 Intermeccanica Roadster
If you own a TDr and are not in the Registry, please go to https://tdreplica.com/forums/topic/mg-td-replica-registry/ and register (you need to copy and paste the link)