Custom display case is what a lot of people need and the their reasons may vary. Some people want to have very unique interior designs with the requisite custom display case to make it a reality. On the other end of the spectrum some people just need to tweak some dimensions on custom cases and therefore require customization. Either way, you will want to find a display case manufacturer with experience in customization of display cases. Often times if you are doing more than changing dimensions, a sketch is a very helpful to convey your ideas to the manufacturer. Even a hand drawn sketch is often enough. The manufacturer will work with you and usually provide guidance on designing your custom display case. Generally lead times are a little longer because there are more steps in the process. Many features besides dimensions can be customized. Among these are finish, which may be a laminate that you like. The number of shelves and lights are another. As well as the general shape of the display case.
Something else to keep in mind is that sometimes customization isn't necessary. Maybe you thought you needed a 47 inch wide display case, but the standard model is 48 inches which might suite your needs just as well. However, if you really do need exact dimensions to fit in a very defined space or if you just need a unique look that is not available then a custom display case will be the only thing that meets your needs. Sometimes it's the item you are displaying that dictates whether you need customization. For example, you may have a statue that you want to display in a pedestal display case. If the size is larger than what will fit in a standard pedestal display case then you will want to contact the manufacturer and explain to them this need. In this case for example, you will want to remember to leave some buffer space between your item and the glass. So lets say your statue is 28 inches high then you will probably want at least three inches extra in height so the statue is not too close to the glass on the top. Custom display case offers a lot of options. The extra effort in ordering one will certainly pay dividends in the long run as you will have a very unique display case in one or several respects.