I'm heavily indebted to Chris Nevard's excellent blog on Catcott Burtle for inspiration on using foamboard as a baseboard, not to mention the various threads on this site where the topic has been discussed - very usefully - at length. So here's my contribution for what it's worth. If theres anything worth noting its this...

1) I used ordinary 5mm foamboard from Hobbycraft. I got 5 sheets at a 10% discount as they were very slightly scuffed at one corner. So far I've used 4 and a half of them

2) Cutting and construction, using PVA to bond, was quick and easy, I think I acheived a lot more in the time than I would have in wood.

3) With cross bracing the whole thing seems pretty rigid, and very light.

4) Despite being very light, and despite my "kitchen table" constraint, I ended up cutting the whole thing in half after construction! At 5ft 6in by 2ft it was intended to be carried up the stairs and stored in the box room, within a very short time I got tired of maneouvring around bannistersa and lamp shades... So - I got an unexpectedly early entry into the art of baseboard joins - not got any photos to hand but 6mm ply end plates, M8 bolts and tee-nuts, and C&L dowels seem to have done the trick. I don't think I'll be building individual boards bigger than 3ft by 2 ft in future.