I own two cd transports that are top loaders.One is the Krell MD-1 and has an acrylic top cover that opens like the hood of a car for example.It requires at least 12 inches of space ,my other one is the Northstar M-192 and it uses a sliding door,so it requires about 6 inches and that's because of my hands trying to load/remove cd's.In general topp loaders should be placed on the top shelf,and that is what I do and I got them placed side by side on the top of my rack.I hope that my answer helps.
Best
George
Best
George