How did I attach windows? I didn't, I mean I made an inner frame of only three sides and then on top of that an outer frame of four sides. The outer frame where there is no inner frame is removable as it's attached with two screws which can of course be unscrewed. The plexiglass pane slides in from the side with no inner frame. Actually the three-sided inner frame just serves as a stop on three sides. The outer frame overlaps the plexiglass one inch all around and actually holds it in place. I do have this set up on both the exterior and interior walls so there is plenty of space inside for hardware cloth to stop predators when panes are out. Also I might add bubble wrap to that space just for the coldest part of winter if it doesn't block light too much.
As to interior, it's pretty plain. An 8ft long six inch wide roosting board held up by sawhorse ends. It's wide so their feathers will cover feet in winter and prevent any frostbite. Then on short side (coop is 6ft by 12ft) I have a 6' by 12" nesting shelf about 14" off ground (resting on large cinder blocks) a 3 gall waterer on a cinder block and a hanging feeder. Straw for bedding. Nothing fancy.
The large yard they go into in the afternoon has yellow crime scene tape crisscrossed across from top of one fence to another -- my idea of something to discourage hawks, which we occasionally see. My husband doesn't like the looks of this so I might try replacing that with CDs hanging from string -- I read that the shiny CDs freak hawks out.