The 100mm Modular Filter Holder from Hitech is a system that will permit you to use as many filters as you need to create the shot you envision. It comes fully assembled but can be modified to accommodate the filters you require at any moment. It removes the traditional limitation of only three filters per holder by the use of the included Plain and Spring Spacers. Using these included Spacers, you can alter the configuration to accept a glass filter as wide as 4mm.
The holder is made from durable nylon yet is lightweight and can fit in your gear bag with no trouble. As an added bonus, it uses the same adapter rings as Hitech's MK4 Aluminum filter holder. This means that you can upgrade your current system without incurring any substantial financial expense.
The Spacers permit you to use glass filters ranging from 4x4", 4x5.65" and 6x4". This kind of flexibility will permit you to expand your photographic capabilities to a degree you hadn't previously considered. When you wish to expand the number of slots, assembly is easy. Once the number of slots you need are created use the included brass nuts to secure the holder together. Additional Spacers and screws are available as options.