I've just been down to B&Q to see what they had. Most of the insulation was as glass fibre rolls, but they did have 2.4 x 1.2m (that's 8x4 to you, Ted
) sheets of expanded polystyrene in various thicknesses up to 100mm (4"). Such a sheet is £20.
So, Ted, I have a q for you. Just how firm is your pink stuff? Does it mark if you squeeze it twix finger and thumb? This stuff does, but it doesn't collapse, just a dent, and not a big one at that. If it's of the same order, I think I might be very pleased.
I'm thinking that maybe a 6mm (1/4") baseboard, with MDF sides shaped to the profile, then filled with this polystyrene, shaped to the profile of the sides and a 3mm hardboard skin over the top would make a robust but cheap and light former.
Next q. How does one cut this stuff without the workshop ending up looking like Santa's grotto in Debenham's? A hot wire would be great, but how to power it? How about a guitar string and a battery charger? Or am I asking to plunge the house into darkness and have my wife dialling 999?
As ever, all suggestions gratefully received.
Cheers
Steve