I had some cutouts I wanted in my screen top and realized the BRS parts supplied only had inside corners, so I designed and printed up an outside corner piece (with a channel for the spline) that works well. The part prints a tad bit less snug than the originals on my settings, but the fit should be good.
For small cutouts like these, you'll have to trim down the length of both corner pieces, and very short runs of spline are difficult to get to stay in the channel (a bit of glue helps), but a drop of glue inside the channel helps retain the corner pieces as well. It's also good not to stretch the screen too tight with cutouts like these, as fully taut screen will likely warp the top.
The model is available here: https://www.thingiverse.com/thing:5397399

