I like the curve skyboard too.
I have curved skyboards and I think they are a mixed blessing. Even 1/8" masonite is hard to curve down to about a 12" or so radius. On mine, I soaked the back with Formula 409 spray to improve flexibility, but it was a single-sided board. Another thing: the tighter curving throws off any impression of distance.
Have you considered the HO Alkali Central from the 1996 MR mag? That didn't have the grades or the figure-8, but was correspondingly more convincing and easier to build. If you elevate the track on 2" foam, you can still do scenery below track level. Perhaps that would an adequate alternative to grades -- sort of a "less is more", but it's up to you

There was another nice 4x8 HO 2-sider in an MR from 1992 tho I don't recall the name offhand. It was reprinted in one of the Kalmbach books. It was cool that it had fall scenery on one side and winter scenery on the other. (I'll see if I can find it.)
Both of those also had roads & scenery that went above/below track level -- a lot of visual appeal.
