In reply to bpmclimb:
I'm in a similar situation - on here reading about lots of great garage set-ups but with no garage - so planning on making something outside when I get the chance. Meanwhile, designing it is a wee distraction.
My backyard fence has conveniently fallen over, so needs to be replaced; equally convenient, the recent delivery of plywood, and the box of holds.
So, the plan here is to rebuild back fence on the wonk, in WBP ply, then add a little a canopy to shield back of wall, or even line out the whole back wall. All those T-nuts are going to expose a lot of ply edges, and there are lots of places for water to get trapped, so my current thinking is to protect the back of the wall with a lightweight skin.
Re. materials - having built lots of outdoor ramps for skateboarding, we found WBP ply as reliable as Marine ply, and half the price. The tanalised timber is a must; I assume you're aware of the need to protect yourself from the dust when you're working that. Also, separating structure from ground moisture.
Thinking about your post, I'm going to hazard an educated guess that 2x2 framing is going to flex excessively. Plying the frame on both sides would help a little but I'd be looking to use heavier framing timber - 2"x6". In terms of design, 4' - 5' wide is pretty skinny indeed. Can you wrap something around the tree, or make a little traversing island?
Thanks for starting the thread. It'll be interesting to see what folk on this one.
EDIT: and there's this...opps... https://www.ukclimbing.com/forums/walls+training/diy_moonboard_in_the_garde...
Post edited at 17:12