Opted to mount them with french cleats -- again, just some scrap pine.

To my dismay, despite taking a clamp audit before designing, I filled it up pretty quickly. However, I could always double up on most of the slots (right now, every clamp gets its own slot except for the 4" Besseys which get 3-per-slot because they're so tiny.) and move the longer clamps to the lower shelf as they take up two slots when on the top shelf.

And I do think I'll eventually add another shelf on the ledge below but I'm pretty happy with this for now. Slowly making the shop more organized and efficient! Next project is mounting my router table between two cabinet sections. This is all part of my "Path to Holtzapffel" -- I really want to build a legitimate workbench now that I've been enjoying the hobby for nearly 4 years and I know what I'm looking for in a workbench -- but I have tons of little shop organization, efficiency, and general-cleanup projects in my backlog. I'm using the desire to build the Holtzapffel as motivation to cross off all these little projects.

The shelves are 8-1/2" by 45" MDF. Eventually, I switched over to using a circular saw (and using the jigsaw to clean up the last little bit left by the arc of the circ saw) as the jigsaw was taking too long to get through so much 1-1/2" MDF.

I used two "slot types". The left 2/3 of the shelf are 1/4" slots with 1" separators and the right 1/3 is based of Jay Bates Universal Clamp Rack -- 9/16" slots in the front half which shrink to 1/4" in the back half, separated by 1-3/4". All slots are 6" long.

Drilled pilot holes, countersunk, and screwed the backing piece on. The backing piece is just some scrap pine I had lying around.

Screws are 2-1/2" and placed every 4".

I have some 1" pipe clamps that I purchased from a friend who didn't want them. I needed a place for them but I don't plan to ever buy 1" versions in the future.

So I just opened up the three far right slots from 9/16 to 1-1/8 on both shelves. The slots will still work for anything smaller if needed.