Here is a fore grip that attaches to a picatinny rail. Featuring a new even shorter version that's only 3 inches long and now all styles are available with and with out the finger notch. Its designed for 8-32 1 1/2 inch machine screws and nuts to fit. Screws will need to be cut down a bit and the nuts are intended to be drawn into the grip when first installed and should remain in place when the screw is removed. If the part doesn't fit or some dimensions are off let me know so I can adjust the design.
Supports only really needed for the rail slot and screw/nut holes.
Print with bridging. Rafts and supports are optional if you think you printer can handle it. 20% to 35% infill provides a sturdy part that still allows the ends to be tightened onto rails. If the middle finger peg breaks too easily you might want to reprint it at a higher infill or increase your shell count.