Just wondering what everybody's solution for this was. In particular, I have a focusrite scarlett 2i4 that would be awesome if I could get into my rack. Could get a rack shelf and Velcro it or something, but thought somebody might have a better idea!
I use a rack shelf (fixed or sliding, depending on what type of access I need once the equipment is in the rack) and drill holes through the shelf to match the screw-in feet of whatever equipment I'm securing. Then I use longer screws run from the shelf, through the feet, and into the equipment. Keeping the rubber get adds a bit of shock absorption. If the equipment doesn't have screw-in feet, I use 3M Dual-Lock - much better than Velcro in my experience.
Rack shelf with Velcro is the way I've frequently gone in the past.
Or you drill holes in the shelf to line up & use existing scew holes on the bottom of the piece. If its racked on a shelf between two other pieces that are screwed in, it will probably stay put just fine with out attaching it.
Alternately, you could just Velcro it to the top of another rack piece & eliminate the shelf.