Question

looking for a horizontal rack mount kit

Answer 1

You need to get the Middle Atlantic Rack Shelf, these are available through any commercial A/V supplier or pro music store. It's a 1U Rack shelf with several dozen holes in it for various pieces of gear. You'll need to take the side panels off the boxes so they'll fit next to each other on the rack. 

Here's a link for Amazon that sells it - https://www.amazon.com/Middle-Atlantic-UTR1-shelf-powder/dp/B0002BG40M