Finally settled on a light configuration on my PS90 SBR. The under barrel 4 slot picatinny rail was purchased from Elite Ammunition. Tried it out with a Streamlight TLR1HL first and while it worked, I didn't care for the feel of activating the light. I prefer the positioning with the Elzetta Alpha with an old Blackhawk picatinny mount I had hanging around in the spare parts box. Just had to move the sling mount back to clear the tailcap of the light.