It looks like I never uploaded this to my old site, even though it was all packaged up and ready to go. It’s a 3D version of the oval window texture from the medieval wad. It’s even got a smaller texture size than the original, although it does have a few more polygons instead! Obviously I can’t take credit for the texture here. In your map, you’ll need to place it 8 units away from the wall you want it to sit against.
Thanks to FifthElephant for spotting the fullbrights on the original version of this model, and going to the trouble to tidying it up for me. A fixed version has been uploaded above.