How much jungle is involved? If height allows, I'd lash it to a frame pack using 1" straps from NRSweb.com and try it in the backyard. The bottom might be too close to your feet for a comfortable stride. If so, use some foam blocks at the bottom of the pack frame to angle it more over your head (which is probably a better center of gravity, anyway).
If that works for you on a trial basis, then I'd put tabs on the pack frame in which to nest the board. They could be the reverse of the "torture tabs" on old Jansport packs. Or you could fab something out of plywood or fiberglass.
For a specific pack, I'd go to a classic frame pack from Kelty but with with a modern yoke suspension system and well-padded hip belts. Usually around $130-160 new or so. Get, or at least look at, the frame extender ("roll bar") to give you more attach points. Consider that you can make something that fits into the frame in the same way, but is somewhat more like a kayak or lumber rack with tabs the width of the board to help secure it.
I go out for a few nights in a frameless, 24-ounce pack holding 15 pounds like the rest of you. But when I'm hauling 30, 40, or 50 pounds, I much prefer a frame pack. Even more so in a hot climate because of the better ventilation against my back.
If you'd be areas with extended miles of low-overhead, then also rig up a shoulder strap (like on a big duffle bag), with webbing wrapped around the board so you can more comfortable carry it at your side when needed.