If it mounts to a drafting table, be sure it doesn't conflict with the lamp or the guides for the parallel bar.

I have a strip with pegs on it from which I hang my templates and triangles; personally, I'd want easy access like that.

What does sixth form translate to?

I personally find it easier to use a rolling cart with drawers and shelves/slots for vellum/trace paper rolls than something that would mount to a drafting table. Problems I would see with something that mounts is the possibility of conflict with a mounted mechanical pencil sharpener, drafting lamp, parallel rule, and if still drafting with mechanical pencils, getting full of erasure dust from an electic eraser. Just my two cents worth, or maybe it just reflects the status quo of what I am used to using...

I did use a utility tray much like the OP describes, but abandoned it in favor of the rolling cart for many of the same reasons bahia outlines :-) Quite simply, the placement was awkward regardless how I had it positioned and it was constantly filling with eraser crumbs, graphite, etc., nor was it the easiest thing in the world to clean.

Now the coffee cup sitting on the top of the rolling cart seems to be the repository for all the eraser crumbs......eeeewwww!!