I tried to heat both sides on the first couple of pieces, but by the end, I could get by with only one side needing heat. It was 10mm thick foam so it required quite a bit of heat, you'll see the slight burn marks in some of the photos.

It was a gradual process, heat and bend, heat and bend, heat and bend, until it fit in the mould without too much force. Once it's a close fit, screw it into place and hot glue onto the mould. Just before glassing, all the screws get removed.

I think it was 80-90kg/m^3 foam. Any lighter and it dints too easily, higher weights aren't necessary on these size boats.

The Airex I used was quite stiff. There is no way of putting large compound curves (bending in multiple directions) into it without heat. I had planned to use thinner foam because it would be easier to form. The relatively flat sides went on very easily. The foam was a little brittle if you tried to bend it too quickly (heated or not), and I cracked it 2 or 3 times.

It wasn't a problem to work with the foam, infact I'm more confident working with foam and glass/carbon than I would be working with plywood. If you make a mistake or create/find a defect, it can easily be fixed.