Matthew, this is the model most of folks use on Cloudy Nights, but they are expensive. It has an optional USB port for adjusting the brightness automatically, and saving the pre-set levels for different scopes or filters.
Personally I use a 22" LCD monitor attached to my laptop in the shed and I use Al Sheenys VLB (Virtual Light Box) software to display a white screen and adjust the brightness. The ZIP file is 224Kb so its too big to attach the file here.
I don't think it matters which brand you get. There are heaps on ebay. Make sure the power is something that is easy to get from your setup, whether it's USB or 12V DC. There are some "high end" light panels, aurora and others, supposedly having a better light spectrum. There also may be imperfections or slightly uneven illumination. TBH some people do sky flats even t-shirt flats and it's working so those panels are good enough for the job. You're focused at infinity anyway. It's unlikely you'd see even a fly sitting on the panel in your flat. I did flats with various brands and make in RGB, narrow band, NIR, UV. Never had any issues with it.
PS: Just saw that link to the "Spike-a flat fielder" at USD250.00. IMHO nice bit of marketing but total BS.
I use a free app on my iPad called screen-light and sit that on a $5 layered foam board from Eckersleys. Works well. The app has some ads that pop up but they are off optical path on my 120APO. I haven’t tried on my Edge 8” but the foam pad is A3 so I’m thinking it will be ok on that too.
