Some firms like 5-Star and Spider might do it, but they may not if they had too many orders that were returned or not paid for.
To minimise this, some firms sell panels and hardware on Ebay, and it's fairly easy to make your own, bearing some details in mind.
Look at existing methods, for example 5-Star tend to avoid complex cutout forms, which decreases weight and raises strength by keeping the edge fittings in single lengths, minimising holes and fastenings.
A rivet gun and rivets are cheap, and buying a case that is close to that you need, removing some parts, cutting size to what you need, then using the removed fittings on the new edges, can work, but it's better to buy the panels ready cut to order, then buy the hardware to cut yourself. A hacksaw and a drill and a rivet gun are the only vital tools. Existing cases will let you figure out what parts you need to order. If you have access to a bandsaw you can do a lot, quickly and accurately.
Existing cases may be bought used and in near-perfect condition, you can dismantle, cut to size, rebuild, with all parts needed coming from the old case, a lot cheaper than buying them separately.
Doing it this way can save a lot of money. If you need new parts, choosing the panel materials and construction, and the right size of fittings to go with them, are the main thing to take care of, but from what I've seen, sellers are careful to advertise these details correctly so it's easy to get it right.
You can modify and maintain rack cabinets at will if you do this. You might be able to get paid for custom work for people you know, too. For this reason, try asking locally because you may find someone in exactly this position if you're not choosing to do it yourself. Ask at practise/recording studios, they probably know someone doing it.