Essentially you need to build a box where the width (inside dimensions) are 19.25" apart, then you bolt in rack rails you buy from a music store or mail order place. You can make it out of metal:
Or out of wood:
Or make them very tall, for a stand-up desk / rack area:
Regardless how you make yours, just keep the inside width at 19.25" and you're good to go.
I took a 5" x16" long shelf from Home Depot and attached 4 pieces. Then added the rails. I attached a crude drawing. It sits on the back of my desk against the wall. Hold 10 rack spaces on each side.