There’s a saying in carpentry “You can never have too many clamps.” This is true until you need to store them. I’ve got a bunch of these cheap “F” clamps which make a messy pile. I knocked this holder together out of 3mm mdf. The slots are 5mm wide and go back 70mm.
As usual, I used the very handy “BoxMaker” extension for Inkscape, though I drew the slots in DesignSpark Mechanical. Note that the top layer (with slots) is doubled up with an extra layer to make it stronger. It might have been OK like that but I later added some simple braces. After that it was plenty strong enough.
Here it is attached to the wall. (It’s actually attached with a French Cleat so I can move it around if I want to).
And here it is fully populated. I screwed another piece of wood to the bottom of the back, a bit thicker than the cleat at the top, so the top now slopes towards the back by a few degrees.
Very quick. Very cheap.