So there are tons of Gridfinity Bins out there, lots of gridfinity drawers, lots of under deck drawers, but no under desk Gridfinity STACKABLE drawers. I wanted something i could eventually add to, instead of just the single under deck drawers i see. I tried a few variations of ways to connect them, including dovetails which worked great, but the narrowest part of the dovetail wasn't strong enough to support more than 1 addon. The longer you make the guides, the more the bottom one can move side to side and not keep the drawers snug. So i opted for snug fit pins. I will probably glue the guides together when i decide what the final setup will be for my desk and my needs.  The beauty of these is you can add to them and\or change the drawer depth for taller bins (as long as you don't glue them). Modularity was a key feature i wanted, but really it was the downward expandability AND gridfinity i wanted. Gluing together eventually is fine.

I created 5 different sizes, 3U, 4U, 5U, 6U, 7U. 2mm above and below each drawer for enough clearance. Each size has a Mounting Top and an extension guide and two different drawer faces. One plain and one as you see in the photos. Each size has a Magnetic grid base layout and a standard grid base layout (no photos as i didn't print).

I created 2 drill hole guides. One for flush mounting, with handle protruding from edge of desk. One for recessed mounting, handle behind edge of desk. Align where you want the drawers, tape the guide (as shown in picture) Either mark the holes or drill with 1\8'' pilot tip bit for best on center start. For the second guide, I inserted the drawer and held the second guide snugly in place with a wood clamp so I could drill with guide in place. Came out great.

Top guides have 3mm of thickness for good strength holding the screws. The holes will accommodate a standard woodscrew head (9mm diameter). 

I printed my guides in PetG for strength and the drawers in PLA+.