Here's an option, and only $16. It would require two holes, drilled and tapped, in the dust cover. Might not be pretty...but it's a P85.
https://www.midwayusa.com/product/1949191715/ergo-picatinny-rail-with-rail-covers-and-mounting-hardware-ar-15-handguard-aluminum-black
It might also be worthwhile to google the manufacturer and see if they have something closer to what you want.
Ted