The flies come in through gaps left by an ancient window ac unit given to us for free.

get some strips of foam or even some hand towels or rags and stuff the openings around the ac unit. Also, if the window is double hung, stuff a towel between the windows at the top of the lower window. Not only will it keep the bugs out, but it will keep the heat out and make the room more comfortable and the ac unit working less hard.

You can also buy spray foam which we use to seal up tiny areas around the house to keep insects and mice out.

Attach a curtain rod from end to end, you can use round rings with clips to attach the netting...you will probably have to cut the netting to do this. It would be easier to seal up the gaps in the a/c etc.

I would forget about the ceiling and attach netting directly to bedframe using tacks on the top of the bedframe. I would cut up the net I had and stretch and tack a square piece across the top and tack a drapey piece across each side. No one will ever see the top of the bed so a few tack holes would not bother me.

Getting a replacement window screen made would be quite cheap.