That looks like a smaller version of the stuff they use for cat walks and such in industrial settings; it's called pultruded fiberglass, although I think it's made with other materials as well (HDPE?). If you search around you'll find sites, and since it's used in flooring for people the specs are available. Not cheap.
And of interest is the fact that folks sell sections of it for use as sand ladders. It apparently works pretty well, although tests I've seen show it to be kinda bendy for bridging use. See http://www.okoffroad.com/stuff-waffleboards.htm
for example. More spendy, but still pretty light (4'x1'x2" was 19 lbs) and you'd have a floor to your rack you could use to get out of a jam. But since they have to be OSHA compliant they tend to be strong but expensive.
McMaster-Carr also sells grated flooring of all different types; you might find comparable specs there.
I keep browsing Craigslist to see if I can come up with a pair. I once thought of making a rack out of aluminum sand ladders; they go for about $100 per 40"x16" length, at 24 lbs. So they might be a cheaper alternative, and just as light, since I assume you could delete some crossbar from the design. The vehicle that give me the idea just had two strapped to a yakima cross bar set, and didn't have anything very heavy on them (a truck, not a SMB). These might not handle the weight of a SMB if they have to bridge anything. I ended up using parts of a modular dog kennel cage; they were powder coated and fit nicely and readily available used.