Over the window you could model a piece that could fit in between the cracks and have a small hole for wire to run down then, make a frame for the pixels hanging from the wire.
Maybe you could almost press fit the frame in? Basically make it almost the exact same dimensions of your window and press fit it in allowing friction to hold it in place. That'll take advantage of the recess there too.
Maybe you could have the same thing from me above but make a coupling on the model so you could connect it to the frame without wind.
The pipe keeps it from bumping against the window, the coupling will go on the outside of the siding with a 3D printed piece on the other back side of the siding meaning no wire hanging down.
Consider a pair of spring-loaded curtain rods or spring-loaded shower rods. Mount on the top and bottom of the window against the sides and attach the frame to it.
Slightly off track. What type of bulbs are those? ST14?
They are ws2811 bullet pixels with opaque c9 covers.
I’m not sure if this would work for your windows because I couldn’t really see the J channel. But my windows have J channel that is easy to access around the outside of each window so I took and some additional J channel and drilled holes in the bottom of the J to stick lights in and then the vertical our back or leg, whatever you wanna call that slides in behind the J channel on the window and just pressure fit’s in there.