By definition, a custom wood form is one that is designed and built for a specific concrete structure. Many contractors find it more economical and efficient to use the prefabricated, modular forming systems now available. Although modular systems can be very cost-efficient for some jobs, they may not be suitable for complex concrete structures. If you evaluate a project and conclude that certain concrete elements or possibly the entire structure requires a custom form, the next step is to determine whether to fabricate the form on the job or subcontract the work out to a company that specializes in building custom forms.