If you want a full-on competition magwell, the Carver Custom is one of the best.
Secured to the gun in two ways, the Carver uses a brass mounting point (basically a miniaturized Seattle Slug) to provide weight where you need it. The aluminum version improves balance and recoil control while making reloads a cinch, but the stainless steel model is our choice for maximum recoil control and durability.
UPSIDES: All "full circle" magwells work like a funnel, guiding the magazine where it needs to go. Using one, it's much easier to reload at maximum speed.
DOWNSIDES: Since this is a "full circle" magwell, it all but requires the use of extended magazines. Without them, it can be difficult to seat the magazine properly, or "rip" a magazine free if it gets stuck for any reason. If you're looking for maximum reload speed, this is your part.
DISCLAIMER: This product is not manufactured, authorized, endorsed, or warranted by GLOCK. GLOCK does not warrant or represent that this product is compatible with GLOCK pistols.