In reply to Ed Booth:
I’ve used a few Moonboards but never had to build one, nor used one in a corner but I have built a home board and I think a bit of space either side allows more flexibility, and bigger moves especially for the finish holds.
how important is it that you can climb all the moon problems? There must be thousands, so it’s unlikely to be a big deal if there’s a few you can’t do as they take you too much into the corner, that would be one option.
If I were you however, I’d fill the space as best you can, with the Moonboard in the centre but the board extending 80cm either side (I assume you can still get to the back somehow?). Then fill the sides, and the gaps between the Moon holds, with screw on’s to make up your own problems.
A variety of screw on holds can be made pretty easily (and it’s very satisfying) with hardwood off cuts from a joinery works and a drill/grinder.