Thanks for the offer. I know how to rabbit the foamcore but this is faster. At some later date the foam edges will all be trimmed in plasticard to add some surface features to the building so the exposed edging shouldn't be an issue forever. I'm thinking about moving to a 45 degree mitre on the corners that solves the exposed foam problem neatly as well.

I think I found a local supplier for foamed pvc so I'm not sure how long I'll be doing foamcore. Probably finish out the 5-6 sheets I have sitting ready to work on then try the pvc. If I can switch over to that alot of my issues disappear including glue time as you can use superglue on the pvc.