Make sure you have the right style case: Many of the smaller injection molded and aluminum carry cases come standard will molded-in panel tabs sets down a short distance from the lid split. This is not true for luggage style cases that are either soft-sided or thermoformed. It is not recommended that you use luggage style cases for panel mounted equipment. Some larger cases (think airline check-through size) also have panel tabs in them. For larger hard cases without stock panel tabs, a workable system can still be constructed, but you will most likely need the assistance of a case professional.
Choose the right panel material: The two best options are ABS plastic sheet and aluminum plate. Plastic has distinct advantage over aluminum in that it is less expensive, lighter, more malleable, and it is easier to route or cut. When using plastic, you will most likely need at least .125”of panel thickness, and up to a .25” thickness for very large panels or heavy items. Aluminum panels can be .0625” to .125”. If you feel that you need more than .125” of aluminum panel thickness to hold your equipments weight, this is a good indication that you should reconsider panel mounting it, and should consider other options such as a custom cushioned case interior.