To be fair, they are probably the same as building any lattice post signal, e.g., LSWR, CR, LNER, etc., so go carefully with the soldering iron and use the minimum amount of solder that you can get away with.
Those 5ft arms are seriously long, though....
I generally use ply or stripwood framing for open top baseboard construction of where the railway is on an embankment.
These days I cut the ply strips on a bench saw and use a jig saw for profile cutting and fascias
My current dock layout uses similar baseboard construction
Re Foamboard, if you know of any firms that make those display hoardings that you see at events like agricultural shows, horsey type shows. The ads are often vinyls stuck on foam board. The offcuts are often quite large and skipped. I got a cab full foc from one company, got lots left and thats over ten yrs ago. Seek and ye shall find!