Here are a couple of pics of my RASA flat setup. I have a Celestron 94014 Lens Shade that I cut almost in half on the RASA. You can cut or leave the lens shade to whatever you need to comfortably cover the camera and its connections that are usually on the back of the cameras so that when you look across the event horizon you don't see any cables escaping. (sorry, cold medicine!) That allows you to put an A2 sized EL panel on top of that making a decent light seal if you are pointing to zenith. I only cut the lens shade because the EL would hit the roof and not cover the objective when pointing to zenith with the roof closed. For clarity, the pics don't show it, but I typically sandwich in 3-6 pieces of easel paper to get it dark enough so that I can be in a 1-15 second range for the flat. This EL is USB powered and dimmable which that helps a lot, but in general I've seen it to be problematic to have less than 1 second subs. For narrowband I use 1-3 sheets of paper, but I don't think I have to get the ADU and time I want, more that the paper sort of helps seal out stray light. The RASA is so fast! I've struggled with leaky flats before, and that's not helpful from a flats correction perspective. I have occasionally added a little weight on top of the EL to ensure it's a good fit the the shade, like an ADM 6" vixen dovetail bar.
Using SGP, there is a wizard I use every time I capture flats to figure out the times I need for my ADU target. These USB EL's don't typically remember what it was previously dimmed to once unplugged. No big deal.
On Amazon search for: A2 LED Light Box I believe any of them should work. Here is the Amazon link to mine that you see in the pics: https://smile.amazon.com/gp/product/B0725Y7H29
I have another brand that is smaller that I use with refractors works well. These Light Box EL's have replaced my wooden framed panel with a real EL and an inverter that also worked great but was much heavier due to the plywood wood backing and white plexiglass. That EL with the inverter was much more expensive than these light box tracer panels. The Light box tracer panel EL was not my idea, a fellow POD owner posted about it here: https://www.astrobin.com/79j7ad (Thanks again Pat!)
Some more on its predecessor. This was constructed of ½ plywood with ¼” white plexiglass and an EL panel that came with an inverter.