I have seen a lot of universal sub-bases so my guess is that someone sells just what your looking for.
While you're checking around look at this post as it has references to a "how-to" on making a sub-base and a reference to where to buy "router stuff online". I could go get the post and list then here but you might enjoy the trip getting there. http://www.routerforums.com/showthread.php?t=460
Why add another base when you can purchase Makita template Guides. As a matter of interest what is the largest diameter guide supplied with the porter cable set? To my mind I believe they are not all the same depth of collar therefore requiring thicker templates for certain sizes.
Tom has a good point, if you get the template guide set from Porter Cable the barrels are different lengths which makes them useful for Porter Cable jigs and fixtures. If on the other hand you want the "pc" standard meaning basicly a 1 3/16 hole size on the sub-base then you might want to purchase a set of brass ones with the short (.234-ish) barrel. They work well with cut outs you make in 1/4" wood/plastic/masonite.
To answer Tom's question the largest template guide I have from PC is 1 1/4" which makes since since the hole cut out is only 1 3/16". The brass ones often go to only 1".
To make the barrel thing a little clearer check the attachment show in front a brass set and in the back the PC set.
To make things even more interesting there is the size that oak-park and others? sell that has a 1 3/4" opening. Then we have the "Tom" standard sizes which are a lot larger. He makes some good points to having the larger sizes so use the membership page then locate his info and look up his posts, so good reading on templates and guide bushings.
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