Have seen racks of similar style to the Arc-Eurotrade one linked to mounted on the wall. They stick out rather a long way if mounted in their design orientation. If you turn it round to reduce stick-out the collets won't sit properly in the holes. Might even fall out. I tried adjusting the (smaller) sheet steel collet rack that came with my DRO system to a steeper angle to reduce the horizontal projection and it didn't work out very well, had to put it back to the standard slope which was very similar to the slope on the Arc-Eurotrade unit. Single metal sheet top plate doesn't give enough support to the collets.
Have seen a "terraced" wall mounting system for collets with single rows of collets angled so collets in each row clear the row above when being withdrawn. Looked effective with minimal stick out. From memory it was a zig-zag folded metal sheet with appropriately positioned holes so most likely a commercial product with commensurate price.
This is a similar idea :- **LINK** but the angles look different. Second small picture shows how it would be screwed to the wall. Appears to have plastic infill beneath the metal plate to support the collets.
This is an Arc stye plate rack arranged to be mounted at a shallow angle on a wall :- **LINK** . Top plate is double with suitable spacing between the two plates to give more support to the collets.
Single thin sheet will hold collets fine if the angle is shallow. Be looking at sides more than end of collets but I don't see that mattering. 4 rows of ten collets would probably work well. I'd cut 3 wooden strips with 4 zig-zags from 1/2" or 3/4" material to support the collet carrying strips. One at each end, one in the middle. Use whatever "not wood" material is convenient for collet carriers. Steel, alloy, HDPE ( which can be a pain to get a clean hole in) or whatever. Offcut of the thick plastic used for soffits ought to work well, I've been saving a couple of feet for years waiting for the right application of this sort.
Clive.