Hi Stefan, this is kind of what I do a lot of the time for my business
Polystyrene... I buy blocks, 8’ x 4’ x 2’
I have a special hot wire transformer, special thin wire and a pair of cables
I make profiles, place them either side of the block and with someone to help me we cut one profile out
Then turn 90 degrees and cut the other profile
To be honest, you can’t do this in your garage. You need space, a hot wire transformer and big blocks of foam
If I didn’t have this I’d do it the traditional way with profiles in mdf like building a boat hull and fill gaps in with foam or plaster
Hope that helps.
When we have a Builders day up here I’ll demonstrate for all
Steve