Same here, Max... Diving "wrist seal-less" is awesome. No fiddly tubes or thumb loops, and donning and doffing is a breeze. I did the same... I cut the suit-side ends off of an old set of latex seals and sandwiched those between the suit-side glove ring and the soft ring glued into the suit that came with my SiTech QCS Oval rings.
Is the QCP (SiTech round) soft ring the same size? Can these Waterproof Ultima rings be used with QCP suit-side soft rings as well, or are the QCP suit-side soft rings a different size?
The last video you posted from Simply Scuba is the one I watched before assembling my rings. In it, the guy assembles a lined glove as an unlined latex glove. This is wrong and will most certainly lead to a leak.
Plain latex, neoprene or nitrile unlined "dishwashing" style gloves can be assembled this way... But anything with a liner - even a low-lint, plain fabric surface - needs to be installed in reverse... With the o-ring inside the glove rather than outside. This includes all black, orange and blue PVC-based gloves from Ansell or Showa such as the 440, 660, the KV660 and more.
...And for anyone following, it is THIS "o-ring on the inside installation" that is giving us problems.
I had so much difficulty with it initially that I installed a set of Showa 707's first, which are very thin, unlined gloves. I installed them with the o-ring on the outside, as is shown in the Simply Scuba video... Then wore a wet overglove ON TOP of that to protect the glove. I simply wanted to see if it'd work.
Worked great... Until I holed the right 707 between where my wet glove ended and the suit ring. Instant flood.
On that dive I still had a set of silicone wrist seals installed, with a thumb loop left poking out for equalization. My whole arm flooded. Had I not had the thumb loop poking through, my arm would have STILL flooded... Wrist seals are pretty much worthless on my tendony, muscular forearms. They leak everywhere, and yes, I think I've tried just about everything... Including wearing them high on a shaven arm and lubed with KY.
...Which is why I wanted dry gloves... So I could finally get dry arms.