In past times I think the MMC used what builders term as Damp Proof Coating, (DPC) which was bitumen coated flexible material, used to isolate low level brickwork on a building from the next layer of bricks above to stop rising damp.. I think MMC used it to allow a degree of flexibility between the chassis and the wooden frame. I also think some may have used THICK tar impregnated roofing felt to achieve the same ends, though being careful to avoid the roofing felts that had a coarse surface, designed to resist weathering on roofs.
Modern DPC tends to be plastic based and does not afford the degree of compressive flexibility of old style DPC...but as Arwyn suggested a dose of Sikiflex or other high quality flexible material applied to the material should provide a degree of flexibility...?
I have no idea where the rubber strip mentioned may be sourced, sorry.
