Having walked across the original many years ago, it was a stone structure with a layer of compacted soil covering the span. You could recreate this in 54mm by utilizing legos (dont laugh) and covering them with either a plaster veneer or with embossed paper.
A number of years ago Joe Porter did a diorama for MMIR of the final sceen in SPR where Captain Miller is firing his .45 at the Tiger. Miller was sitting up against a damaged motorcycle. Very realistic.
Anyway Porter started with legos and built a roughed out bridge shape and then covered it in sheet plastic and finally embossed paper that looked like stone. for the top of the bridge and some details he used an epoxy putty.