I would like to add a short rail section to the standard slimline forearm on my PTR-91 as see in the pic below.  GG&G has their Under Forearm Integrated Rail for the M4 that secures to the standard handguard and am looking for something similar.  Anyone have something like this?

I would just take a magpul rail (like the ones that come with the RVG), drill 2 small holes, and bolt it to the HG.

I figured on having to do something like that.

That's what I did to mine, didn't want to hack on the original handguard so picked up a surplus handguard for $8, filed part of the rib off the bottom and drilled two holes. Both bolts run thru the heat shield and the rail is solid enough to mount a bipod. It's a simple job. If you don't already have a rail to mount you can get a cheap set of UTG rails from Amazon, either a set of three 6" rails for a G36 for $16 or a set of two 4" and one 6" for $15. If you're looking for something nicer try Midwest Ind.