You only need one topper and maybe 8 bails of straw. Use the straw like leggo blocks in the same size as the topper, put 2x4's down accross for temporary perch, then set milk crates where the big opening window is going to be right on the ground, and set the topper on. It will work easily for a temporary solution. Good luck with getting the girls to go in on their own though. At first they will be really confused.
My birds have a couple truck toppers they use all year round for dust bathing or hanging out in when it is raining.
I've been thinking truck caps would be an excellent coop roof. I've found 2 sources for FREE oak pallets. Now I need a truck, to gather up my supplies.