This Tentacle System kit includes a base and a grip that each attach using 4mm hardware.

The nut fits inside a slot and the bolt attaches to it through a hole in the bottom. The washer is optional. If you use a washer, it would go on the inside of the object behind attached, just under the bolt head.

Don't over-tighten the the bolt. Over-tightening can cause layers to separate and cause a crack by the nut.

You can learn all about the Tentacle System here: https://3ddoodle.com/tentacle-system/