Not exactly "making my own foam board" but I have had some success scratch building with XPS laminate flooring underlay.
In the UK it is avialble in 5 mm thick sheets in grey or green. Any store that sells laminate flooring will sell underlay.
As you can see it is quite a dense XPS with a passably smooth surface. As a result is is fairly rigid but snaps readily if bent too far. It does have a slight "grain" effect but nothing like that of original Depron.
It limitations to bending does mean planking can be the only process to achieve a sharp radius but its rigidity allows some structures to need no reinforcing.
This scratch built 1040 mm span Hawker Sea Hawk has a 55 mm EDF with an 1800 mAh 3s.
It uses a 3d printed bifurcated inlet and exhaust duct but the rest of the structure is just 5 mm XPS foam skin over XPS formers with no reinforcing at all, even in the wings.
A rather tricky build but the end result means it is big and light enough to only need modest power, so scale inlets and exhausts, and it flies slowly enough to be an easy hand launch and bellyland on grass.