Personally, I wouldn't clamp anything to the gas block. I think the best way is to make an extension with the exact profile of the factory rail and then use some sort of coupling clamp that spans two slots (the front one on the HK rail and the rear one on the extension) and locks the two parts together. You'd lose a couple of slots, but most people don't use every slot anyway. You might not need to lock all 4 rails together. If the rails mate up tightly, probably just two, top and bottom, or right/left might be enough. This way, you can remove the whole rail/extended rail package as one unit like now. If you start clamping things to the gas clock, you'd probably have to remove it to take off the HK quad rail to access the piston for cleaning.