What you have will probably work. It's way oversized but that's not the worst thing. Feeding-based sizing is 12 sq in lit on both sides per cube of food per day, with 6-12 CFL watts per side (18 hours/day for the lower end, 9 hours/day for the higher end)
Sizing it based on feeding would put you down to a screen that is more like 4x6 or 6x6. But you will get the majority of your growth under the coverage of the dome which looks like a 10" diameter.
Lighting is actual watts, not the equivalent, so those are 23W ones, correct? Going by the CFL guidelines, 23W on each side would be 46 total or roughly 4 cubes/day "capacity" when lit for 18 hours/day. So if you then re-sized the screen to match the lighting, that would be a 48 sq in screen or 7x7 ish.
The only effect you might see with a oversized screen when compared to the lighting available and how much you feed the tank is that the growth might get spread out and night not grow GHA quite as well. If you shrink the screen down to match the feeding and lighting, this encourages the growth to be in a more condensed space and that tends to result in more GHA. But that's not a hard-and-fast rule necessarily so I wouldn't change a whole lot. You might make the screen a few inches narrower and run the same flow, which will mean your flow rate per linear inch is increased which is always better. Then just leave the screen height the same so your drainage into your sump is smoother (i.e. no water falling off the screen and crashing into the sump)
The other thing with the oversize screen is that it might initially grow very well, and then as your tank chemistry slowly changes you might see growth start to diminish. That would be a sign that you might want to downsize or change the lighting photoperiod, etc.
The larger pipe slid over the main slot pipe is a good idea because eventually you will get growth at the slot/screen junction that will cause a streamer. You need to prevent this and protect the lights (as well as keep water IN the sump!). Rigging up a piece of acrylic or something between the screen and the lights will help protect the CFL tubes also but you don't want them in close proximity with acrylic as they can get hot.
Overall, good simple build!
FWIW I just ran a battery of tests on 4 tanks the I run scrubbers on, 3 of them scrubber-only long term and all 4 tanks have nitrate 0 phosphate 0.05-0.06 (one was 0.14, but it's severely neglected and no coral). No water changes for a couple years on all of them...once a scrubber gets trucking along, it really does the job. The one tank (200g) that has other filtration also has lots of big fish (like a 10" Vlamingi and 3 other tangs) and a skimmer (RO 150) a filter sock and GFO/carbon but I don't know how often he changes it.
