If you're looking to build a rack mount PC, there are plenty of rack mount cases available, just type it into your search engine of choice.
Pay attention to the dimensions though, some network cabinets aren't very deep so you'll need to measure up and check any case before you buy it.
Also of note is how many 'posts' the cabinet has (The amount of vertical posts that you attach the equipment to). These most usually have 2 or 4, if your cabinet only has 2 posts then the weight of the PC could be an issue.
Or, If you want everything on display and not in a case, then you could buy a network cabinet shelf and drill the holes for the motherboard into it.
A vented shelf would be best, but may cause a problem when you come to drill holes.