Failed twice printing this, entirely my fault. Finally tried in my favorite color filament, printed beautifully. Underside is much rougher, but not horrible. Fit flawlessly in 6 different upper receivers that we tried. Seems strong enough, think you'd damage the upper before you broke this block. Printing on the side is the key, with thick enough walls and enough infill.
1.2 mm walls, 3 perimeters. Printed at 220, bed 60, supports everywhere. Approx. 12-hour print time.