Help with rack dimensions please?

I am in the process of getting a quote from a local plastics company for a small five slot rack, and I'm not sure what dimensions to give them. It would hold Rubbermaid 4221 bins, their dimensions are 22.9 x 16.8 x 5.9". It would basically be similar to this boaphiles rack.
http://www.boaphileplastics.com/rhin...sterilite.html

I will most likely be using either 6 x 11" heat mats, or flexwatt.

So what I'm unsure about is how much room to give on each shelf? I was thinking of making them 24" deep and 19" wide. Is that a good idea to give it a little extra room on the sides and front and back? Or should they be more snug? And how much extra room do I need to give in height for each shelf if the tub is 5.9" tall? How much room do I need for heat mat and air flow? Do I have to raise the bin up off the heat mat? If so how much? I'm trying to figure out what dimensions to send for the quote, but I royally suck at figuring this stuff out.