mary quilting

Foam Insulation Board and use duct tape I used l L-Brackets and pushed nail threw the board in to the L-Brackets

fireworkslover

I used 1" thick styrofoam covered with batting and hot glued in place. Then cut my fabric and used straight pins to hold it in place, thinking I might want to remove it sometime to launder it. I installed "L" brackets on either side of the window frame to hold the whole thing in place. They are lightweight even if 48" wide.

Rose_P

I made a couple of these from kits that came from Hobby Lobby, but other places sell them. They have pieces of styrofoam molded to a shape and you wrap the fabric around them and push it into grooves. It's been about 8 years, and I don't remember the details, but they were easy with good instructions. I'd recommend not getting very thick fabric. Drapery weight is okay. I used a couple of remnants because they didn't need much fabric, and there's no batting. If it looks dusty in this picture it probably is.

Rose_P

I meant to say that you could definitely use a patchwork piece made of quilting cotton for the above design (in the middle part), but you probably could not use a layered and quilted piece unless you made the quilted part only as wide as the part that shows and left the top just enough wider to tuck in, if that makes sense. That could work okay, probably. You can change the fabric later if you don't use glue. These came in different styles. I googled just now and found the website of a company that makes them. http://www.cornicekits.com/index.php?page=cornice.inc They sell expansion kits to make them wide enough for a bigger window, and if your window is smaller, there are instructions for cutting them to size.
Also, Joann's has some made by June Tailor, but I don't know how they compare.