Or.... look after plywood in the elements.....
I built my obs in 2007, and it has seen relatively little use apart from a year then and another couple of years 4-5 years back.
I recently gutted it out to refit the roof with track and steel u-groove castors. This required removing one end of the obs, so the job was extended to some much needed maintenance.
In fact over the years it hasn't done too badly - with only one area of significant rot in the plywood, it turns out my roofing skills weren't fully up to scratch, and the end flashing left a drip-drip at the junction of some ply sheets, where I thought heavy paint would keep them protected.....
So now any ply end-grain that isn't fully sheltered is snugly enclosed in aluminium channel and/or angle, sealed in tight with generous beads of polyurethane sealant.....