I bought two tortoise "P-bass" pickguards from Guitarfetish, and neither one fit my 1993 MIM Precision or my 2011 Highway One Precision. They were so far off in fact that they didn't even cover the electronics cavity completely! It looked like someone had mis-cut the template badly at the Chinese factory. Plus, the tortoise pattern was pretty awful... like an out-of-focus photograph.
I was so upset at the poor quality of the cheap 3rd party approach, that I went to the other extreme and bought a genuine Fender nitrocellulose re-issue tortoise pickguard from Angela.com. It's the best looking tortoise pickguard I've ever seen, and it fit perfectly, although it
does arrive "curved"/convex at first, apparently due to the nitro finish process where the nitro layer shrinks more than the other layers. It flattens over time once it's screwed down.
Of course, it was a lot more money... about 5X more to be precise. But good tortoise pickguards are apparently not easy to manufacture.
http://angela.com/fenderstandardprecisionbasstortoisepickguard.aspx
I think the StewMac pickguards are also supposed to be good fits, and as long as you avoid the tortoise patterns, they'd probably look just fine.
Oh, and by the way, you could try to find a template for the official Fender pickguard and print it out, and overlay yours to see if it matches. I bought a Fender thumbrest for a few dollars and it came with a full-size template to show precisely where to punch holes.