Standard Porter Cable Base Plate
This is the standard Porter Cable base plate for most 690 & 890 series routers.

This base is mainly used in conjunction with Porter Cable or similar generic template guides for pattern work. Porter Cable has the industry standard for template guides & there's a good reason... they work.

The center opening is a bit small for anything over a 1 inch diameter bit, so if you're going to be using a large roundover, chamfer, ogee or any other large bit, you're going to need another sub base. Large opening sub bases can easily be made from lexan or polycarbonate materials of 1/4" to 1/2" thickness.

These bases are specifically made for Porter Cable routers, but with a little engineering, you can adapt it for use with other routers.

This base is black with some holes for viewing your work, but I much prefer a clear base.

If you do a lot of template guided routing, you need one of these bases (along with a good set of template guides).

Alternative to pricey 2-1/2" hole base
This is a good alternative to the more expensive clear base with the 2-1/2" hole (that item was backordered and this standard base plate is usually available for less). Just enlarge the hole so it's big enough for larger bits like the the 1/2" roundover, and you can use all the larger 1/2" shank edge forming bits that wouldn't fit through the standard base plate hole. There's still enough meat around the six outer holes if you enlarge the hole to about 1-5/8" with a forstner bit or a holesaw on a drill press. It won't give you a huge hole like the 2-1/2" model, so you can't use a big slot-cutter, but you'll save money over the specialty clear base plate.