Cutting the rocker slices out of the XPS foam is the same process as gluing up balsa to make a solid balsa surfboard. They cut a rocker out then glue up the wood then shape it.
Do those wooden rails have the rocker cut into them at this point?
Standup paddleboarding is sweeping the lineup.
Are you asking me about the wood in the photo? Those are rocker templates I used to hot wire foam with. I have 3 or 4 sets of them between 7 and 8 feet long. I haven't had much success hot wiring slabs so I don't do it anymore. I think I was getting the wire too hot. It would always break then be stuck in the foam. I decided that I could get more out of a slab of foam by cutting rocker slices and I use a jig saw or a hand saw to cut those. I cut the tab on a sawzall blade to fit my jig and that works well for thick foam. I also found some longer jigsaw blades that work for foam up to about 4 inches.
I hear you on the hotwire thing. Even with help pulling the wire through the block I get places where there is a hole burnt because we paused, or a jagged cut where we got the speed wrong. It's easy to drift off the wood guides if you're working by yourself.
There was a tip to use those little steel thread bobbins as rollers for the hot wire on the wood forms, but I find they jam if I screw the wood tight to the foam block.
I use steel fishing trace from the game fishing shop. Heats up well and doesn't snap easy.
Hotwire is easy to build.
Not sure about finding the materials in Vietnam but, if you can find a variac, you're in business...
Using a hotwire could not be easier.
Just keep it moving and keep tension against your rocker templates.