Does anyone have a recommendation for keyboard trays mounted under the desk? Not the QWERTY kind, the MIDI kind, so something big enough to hold a 49 key controller 32" would be great, but I am interested in whatever.
I can drill (it’s a 41" Alvin drafting table), but clamping might be nicer.
I love the custom designs I see here, but I am too unskilled to build it myself, and too cheap to buy one 
Thanks!
You may not like this answer, but you’ll want it to rest solidly when you press keys and so hanging out front of a light Alvin table like that… I would bet you’d dislike as either uncomfortable, wobbly or both. Maybe instead a light and very cheap Z stand that fits under the desk to hold it? It Won’t really take up any more room underneath than a moveable shelf would and it gives you placement options.
FWIW, I have a heavy custom desk with midi keyboard tray, also a heVy two tier X stand, and a lightweight cheap z stand like I mentioned. The desk is the hands down best for writing, though I needed a drafting chair with a wide range of height to use the top and keyboard without my back hurting. I bought the just about weightless z for travel gigs as the “pro” X stand is bulky, heavier, and takes more room. But of the three, the X stand is my favorite for playing comfortably, best adjustment, and doesn’t wiggle like the z does, especially on carpet. But the Z would fit better underneath or to the side if my room was set up that way.
That’s interesting, I hadn’t thought about sneaking a detached stand under the table. The table is actually quite solid and heavy, though good points about the pull out tray – even if it’s just a little wiggly under pressure it would feel weird. Plus I can maybe get 49 keys which would be ideal, I don’t think I can fiind a premade pull out thing wide enough for more than 37 keys.
You could consider the VIVO MOUNT-KB15EB keyboard tray available from Amazon. The tray is 33.9" by 11", and the total length including the clamps is 39.4". I own a narrower version of this keyboard tray for my 37-key MIDI keyboard and think it is very sturdy. Just be sure to do your homework and make sure there is enough clearance for the height of your keyboard; the thicker the desktop, the less clearance is available. My desktop is quite thick and my keyboard fits with just a few millimeters of clearance to spare.