I built a light box to allow exposing flat frames inside the observatory with the roof closed. The box is constructed of foam-core panels held together with hot-melt glue. It's attached to the west wall about two feet from the parked telescope pointing directly at it.
I can switch the light box on and off remotely, using an Ethernet power controller. Now all I have to do to take flat frames is to park the mount, turn on the light box, and start exposing.
LED under-cabinet "puck" lights are glued midway along the side panels next to the front panel. They point toward the corners, where triangular reflectors help diffuse and spread the light evenly.
Even with the corner diffusers, I had to fine-tune the illumination on the rear panel. As seen in this photo, each LED has a diagonal reflector plus a black square affixed to the front panel to make the light intensity in the center of the rear panel match the intensity in the corners. Using Pixinsight to measure, I was able to achieve a "flatness" of about 5% over the area visible to the telescope.
The LED wires run through holes in the front panel, and are glued to it on their way toward one corner.
The back panel is held in place with Velcro straps. It's easy to remove for repairs or modifications (hopefully infrequently).
Updated May 23, 2023