Sounds like a reasonable plan. I hope it worked out OK for you.My plan for this weekend is to get the ply needed to make the sides of the roost box and have it cut to size, another right angled bracket, and two more ornate brackets for the second roosting bar. I also need to visit a distant fodder store to stock up on probiotics.
Time permitting, I'll see how well the ply sides fit by clamping them in place and if needs be, I'll make any slight adjustments with the jigsaw. Then it can be taken down again to be painted on the outside and have its lino stuck to the inside.