I've got an extra AR lower that I'm itching to do something with and I've been thinking about making a PCC.  Are there any good magwell conversions for Glock mags, ones that will lock open on the last roun, and is there a particular upper that I would need to use to work with a particular magwell adapter?

There isn't a commercially available magblock for the Glock Mags yet. I've read of a few home tinkers that have produced them, but nothing for sale as of yet. A dedicated glock lower is your best bet for now.

Here's a possibility, but until it's full tested I wouldn't recommend it.

Glock Mag block from Facebook

As near as I can figure, the cost of a dedicated Glock mag lower is close to what the cost to produce a Glock mag magwell adapter for a standard AR magwell would be.  This belief is based, in part, on the demise about 5 years ago of the VM-Hytech magwell adapter, which, if you don't know, is an adapter that uses unmodified Uzi mags.  The "technology" involved for the magwell adapters are very similar.  So, just as the dedicated "Colt" 9mm lowers doomed the VM-Hytech magwell adapter, dedicated Glock mag lowers foreshadow the failure of any attempt to successfully (i.e., profitably) market a Glock mag magwell adapter for a standard AR magwell.

I looked into a putting together a DIY Glock mag adapter, but as I was able to put a dedicated complete Glock mag lower together for less than $250 (not including the cost of a suitable buffer which would be required with an adapter anyway), the cost, time, and trouble involved in making an adapter wasn't worth it.

MHO, YMMV, etc.

In my mind, the only appeal to a Glock mag well adapter over dedicated would be for people who have a registered SBR standard lower receiver already.

I saw that when I did some searching.  I've also contemplated using a dedicated lower.  Just getting an idea of what's out there.