I was actually talking about the quad rail SIG 556 Pistol.
But to answer your question, some people have started cutting up a quad rail from a different gun (can't remember the gun right now) and it fits perfectly.
But if you just want to add one or two more things to your PLR sides, PS90 rails also work. But you will need bigger washers to secure them in the PLR forend.
The holes for the PS90 rails line up in the PLR forend holes just right.
i had a kel-tec su16c i liked it, always thought the pistol was kind of a cool pdw idea. the only reason i sold my su16c was to make room for my second ps90. The sig looks pretty cool as well i wonder how much it will cost.