Everyone is different, but I would find hotwiring a foam core and doing a layup on top of it, then removing the hot-wired unneeded channels as more achievable than building two female molds, fabricating the wing skins in them, procuring EPS kernals, and injecting steam into the void to get them to puff up exactly right. I don't have a large quantity of available process steam at home, and making that happen would be some high adventure.

There is EPS that is half the density of XPS, but it is much less than 1/2 the compressive strength of the "normal" XPS foam used for solid core wings. ASTM C578 Type V111 EPS has a minimum density 1.15 pcf, but it has a compressive strength of 13 psi (with 10% compression). It is very soft. You
can get 60 psi EPS (it would be ASTM C578 Type XV), but it has a density of 3 pcf (about 50% more weight than comparable XPS).