We built our own racks, too.
Once we got our flatbed, it became quite obvious, we could no longer stock flat substrate upright in bins and whatnot. If they weren't used within 24 or 48 hours, they'd bow and it was sometimes tough on the vacuum of the flatbed. 3mm acm is one of the biggest culprits of this, with Cor-X right behind. So, our main set of shelves are 5' x 10' and about 9 shelves high right in back of our flatbed, so they go right from the shelves directly onto the flatbed. They're about 10" high so we can fit a fair amount of stock in there. If we get a big job, we have two rolling gurneys we can put maybe 150 sheets of stock on each.