Thanks for the advice, especially making it as wide as possible and for skipping the Moonboard setup.
I think what I'll do is build a 2.4x2.4m wall at a 25-degree angle - essentially the Moonboard spec (minus the top panel) as I had planned, but populate it with random larger/more positive holds. If I get super strong, maybe I'll invest in some Moonboard hold packs later on - at least I have that as an option. I'd have gone wider, but I don't really have the room in the garage. I'm warming up to the idea of setting my own problems - though any tips or pointers to online resources would be most welcome!
Moonboard holds turned out not to be an option anyway because they are currently out-of-stock (though say they expect some more in 6 weeks or so). As I started to shop around over the weekend, seems quite a few places were out of stock. In the end, I was almost panic buying holds! I ordered various packs of holds from 4 different online places and glad I did as so far had only 2 deliveries - fortunately I think I have enough (~50 holds, 10 screw-ons for feet) to populate a wall. I also took Monday off to go round and buy all the timber I needed. I was almost going to get Travis Perkins to deliver the timber, but opted to haul it away myself in my camper van Monday afternoon - glad I did!!! So, I have all the bits I think I need, just in time for the lockdown. Phew. Now I just need to build it...