Have you ever considered using lightweight cement boards?  There are several brands available at most big box home improvement stores... on average it costs under $10 for a 3'x5'  panel... combined with PVC "Tuff Board" (or equivalent material) spacer and frame parts it could make a fairly durable, maintenance free structure.

Both Lowes and Home Depot carry light weight backer cement board (used for backing tile installations)... Mostly a 1/4" thick panel weighs about the same as a 3/8" piece of OSB.

The PVC board is more expensive than wood, but is fairly durable and has decent longevity, especially when painted.

Overall, it would probably cost twice what wood would cost and be slightly heavier, but would last three times longer, with no maintenance.