Try the Filter Dude. I use his 100mm lens holder as well as the Lee holder. Yes, the Lee is nicer but at $25 for the Filter Dudes, you can't go wrong. He sells quality hardware. He also sells the adapter rings. All my adapter rings are by Filter Dude. If you're shooting 24mm or wider on FF, get the wide angle adapter ring. Also, he has a great lens cap that goes over the adapter ring so you don't have to keep removing it every time you take the filter holder off. Get a few, they're cheap and well worth the cost. His URL link is below.
FilterDude - Affordable Photography Accessories