Steve, Kate,
I find that the diffuser (or simply a sheet of artists quality tracing paper taped to the end of the tube) works well for twilight flats. A mate of mine uses a clean white thin t-shirt!
For more formal flats I constructed a lightbox out of white (artists) foam mounting board from Hobbycraft. A couple of A2 boards was enough for me.
It’s very light (no pun) and can be cut with a craft knife or Stanley but is really rigid and can be glued with PVA. I constructed mine in two halves. The first contained a (tracing paper) screen which fits the end of the scope with an outer ring of LEDs powered by a common switchable 12v adapter supply. The LEDs face outward and do not directly illuminate the screen..
The LEDs work down to 5V so by switching the adapter you can change the brightness.
Skyflats need just this unit.
If no ‘sky’ than I put another foam box over the end and turn the LEDs on. The box internally reflects the light so as not to produce any direct light or shadows onto the paper. Evenly illuminated without any interference patterns.
Works a treat.
Regards
T