Been a bit distracted to post so here's a pile of updatery...
Made a couple of casts and did a dry fit with the arch...
...joined it all up and started filling and reenforcing...
...building this so it can take bases, this is 40mm wide, always been irritated by 6mil bridges typically being too narrow to put a base on especially if you're fighting over a bridge you want to get on it - don't you?
Bit of fiddling to check angles so bases won't slide...
...yep that'd work.
Mask off the sides to fill the road bed with regular filler with PVA mixed in...
Rob from 2d6 messaged me about buying the master, which I'm only too happy to sell, more info on that:
https://leadadventureforum.com/index.php?topic=123105.0Works well, would suit 10mm as well I think, that's 6mil cav in the photo...
Now to make a ruin... should have read the instructions.