Several years ago I purchased a Mec-Gar Plus 2 magazine adapter for my Beretta 96D. It made the 13 round Mec-Gar mag a 15 rounder. I recently discovered the baseplate fits my SW9VE 16 round mags perfectly giving me an 18 round capacity magazine. I would suspect they would also fit the SD9/SD40 magazines. The plus 2 adapters sell on EBAY and amazon for about 13 bucks.

Mec-Gar Optimum Series Plus-2 Adapter F42099 765595510173 | eBay

Have you tried the new aluminum MecGar +2 baseplate on the SW9?

Apex makes one specifically for the SD series. I have one but I prefer the factory baseplate configuration.

No, I haven't. Didn't know Apex made baseplates. I'll have the check that out.

I have run the SD9 series with the polymer MecGar +2 extensions. They work but are just a touch loose. In fact almost any M&P extension will fit loosely and work in the SD9 series mags. Not something I would use for serious work.

I was also a "beta tester" of the Apex extensions (the green ones) and they of course fit and run perfectly, but they are also a bit expensive and getting harder to find. They are your best option for carry and self defense use.

The polymer MecGar extenion fits my SW9VE magazine very tightly - no wobble. I thought the SD9 mags were the same as the SW9's except for the base plate. Guess they are a little different.

The looseness is not side to side, but front to back. While mounted I can push the extension forward or rearward a small amount. I have them on a 16 round SD9 mag and a re-purposed 16 round SW9VE mag, both do the same thing. I did have one pop off the SW9VE mag when doing administrative reload mags on a range with a concrete floor, so maybe the specs are minutely different.

Just checked mine. Very little movement front to back - maybe 1/16 of an inch. No movement side to side. I've had more play in OEM mags/plates (Ruger's comes to mind). That said, I'm not sure I would use these for anything more than practice until I had used them for a while.

I used a +2 mag extension off a canik tp9fsx 20rd mag for Sd9ve. Just took the spring and extension and put it on the SD’s mag. Works like a charm. Feeds, fires, and no hiccups.