Make it fit. There is this problem these days. That people dont make it. If they cant buy it that way. Then they cant have it. I have made all kinds of things out of other things to fit what i want.
I took several foregrips and modified them until i found the one that fits right. And likely will modify something else again and change it to make it into something else after i have used it a while and found something to change or make better.
Hi-tech pic rails modifed to make fore grips that arent cheap plastic. And still give me a place for my light mounted @ 6oclock. -(this one didnt tig weld good because the alum they use on one of them is beer can tin. But i spool gunned it... eh. It has held up so far.)-
Anyways. Make it. If you have a idea. Make it happen. Even if you have to get something and modify it.
I think the OP's point is that he bought something that was supposed to fit, but it doesn't. People shouldn't have to modify products to "make it fit". If you buy something made for a purpose and it doesn't fit, that's on the manufacturer.
Looks like the company is headquartered in Germany and has a U.S. operation. As noted above, the foregrip is designed and sold as an AR accessory. It goes to show that just because something can screw on to a rail doesn't mean it's actually meant for every application that has a rail.
I too would think the extended aluminum lower rail would make it fit. It would fit the cross bolt placements on the grip which is most important. It would also give it more durability. Even though I have the simple Magpul vertical grip which works great, out of all the other's, the Hera would be one of better choices for the KSG.