Although off-the-shelf cases with foam kits are suitable for some uses, a custom case interior such as South-Pak's FoamFrame™ is often more economical in the long run. Cases that are well designed can enhance protection, organization and appearance. The choice of an appropriate cushioning material is paramount in the design process.

The primary cushioning materials used for custom cases are: flexible polyurethane foam (both open cell and closed cell), polyethylene foam, and cross-linked varieties of both these polymers. The foams can be made with antistatic, nonconductive or flame retardant properties. They can also be made in a variety of colors and appealing textures when presentation is important.

While density (pounds per cubic foot) is the most fundamental foam property, firmness, resilience and compression are most important in case design. Polyurethane is soft and spongy and is susceptible to punctures and tears from small or sharp objects. Polyethylene is firmer and thus capable of sustaining heavier loads, but it can be permanently dented (takes a "set") when under constant load. For this reason, we often utilize a variety of foams in the FoamFrame interior depending on the particular stresses that are likely to occur in different areas of the case.

Often the items inside a case need as much protection from each other as they do from external forces. If the separate components within a case are designed to operate as a unit, the cushioning between the items can be reduced. We often use hard ABS plastic sheets covered with a high tech nonabrasive material to create this "internal unit" approach and effectively reduce case size while maintaining adequate protection.

Most foam kits are made only of low density polyurethane and therefore a much larger thickness of material is needed everywhere in the case to achieve the same protection provided by a custom designed case.