Like the Springer Precision variety, etc.

What's out there on the market, which ones expand capacity? If I have a 17 rounder, what all do I need aside from the baseplates?

Obsidian Arms makes a bunch. None of them expand capacity.

https://obsidianarms.com/produ.../magazine-base-pads/

 
I have the Springer Precision aluminum baseplates on all of my 320 mags. I bought the 1/4" version, rather than the 3/8" one. 1/4" is plenty to "stick out" to give you a clean insert. Even with their magwell installed, it is still plenty, and it all fits in the IDPA box. Neither adds extra capacity. For an extra $1 per baseplate, they will laser engrave numbers on the bottom for you, sequential if you wish, or any other sequence (number them all #8 if you wish).
 

Springer is really the OG in the business, and probably the most recognized (along with TT) in the competition circles. And he is a really nice guy. Some of these other companies may have great products but I trust Springer a whole lot more than some of these day before yesterday companies.

You can also get Springer baseplates that are engraved with the Gray Guns logo.

Studies have shown that having the GGI logo on your gun makes you shoot 13% better.

https://grayguns.com/product/p320-25-basepad/

https://grayguns.com/product/p...ed-capacity-basepad/