I have mine in a shed, and when we first put my rabbits in last summer, with the fixed window, the shed got WAY too hot in summer. You definitely need to open the window or take it out. I couldn't keep rabbits in there, I was terrified they'd die.
So I took out the perspex window and I put chicken wire over the window. What I did was stapled some to the outside of the window covering the whole window (I have cats who were trying to get in and nearly did, so use lots of staples or nails!) and then I stapled some chicken wire to the inside of the window, only covering the bottom 2/3rds, so I can slide the perspex back in between the two bits of wire and it stays in place.
I put the perspex back in at night and all through the winter. It's not too warm yet but I take it out during the day to give them fresh air, cos it's not too cold either. Wire mesh would be stronger than chicken wire, I just happened the have chicken wire with small holes already so used that.
There are other ways you could hold the perspex in place but that's just what I did
Oh and get a shed with lots of windows if you can
I only have one window, so it still can get a little warm in there, and there's not enough light. It's fine really, you can see fine, just not as bright as outdoors and not as much air can get in as I'd like. But I already had the shed, so had no choice on how many windows it had.