Many folks are going to say Holosun, and they have a point. However..........I HATE Holosun. They are a direct supplier to the PLA, and the US market buying, using, and doing R$D on Holosun Products over that past decade or so has led directly to improved performance of PLA issue optics. Even on something like a red dot, which is mature tech, I can't urge people to support that company.
But when someone inevitably recommends a 507, it's not a bad dot, just a crap company.
That said, there are a ton of good kinda cheap dots out there. I like Swampfox for my cheap chinese made dots, but I get a significant discount of MSRP, so that helps. Vortex has the Venom for sub $200 that has been a favorite competition starter red dot for years. Their Defender ST is a more "duty" built sight and is tougher. Can be had
here for $250. Primary Arms makes some Micro red dots in the Mid $200-mid $300 range. Their "classic" series red dots are pretty meh, but the SLx and GLx lines are pretty decent values. I have a Sig RomeoX that I like, but the G19 sized one is getting close to $400, which while still a decent value is edging out of "budget" red dot.
IMNSHO, once you hit $400-$420 ish, you are spending enough to just stop, proceed straight to Trijicon, do not pass go.
On mounting, they definitely doo make plates that you can slide in the rear sight dovetail. They are on Amazon in a variety of configurations. They aren't ideal, at the dot sits pretty high, and there's some leverage issues. IIRC you have a mill as well as a lathe? If so, I would mill the slide. I've done a couple, and I don't mill in the posts the manufacturers use, I just mill the slot in the slide to a non wobble fit so the slide holds the optic, then drill and tap the screw holes. On a Glock, the left hand screw hole will hit the extractor plunger channel, and depending on how deep you go, may require you to cut a custom length screw. It's still only about 45 min of work.