If you are going to leave the dark slide in (and have a good way to indicate exposed film without reversing the dark slide), then black velvet or felt should suffice, though the nap will wear and crush over time. Dark slide seals have to both protect the film when closed, and prevent light bouncing around the camera when on the camera and withdrawn.
Commercial film holders usually have at least one sprung side (using finger stock) to keep the light trap closed.
If you are working to the ANSI dimensions, there is not a lot of space between the camera back reference surface and the film to install a spring light trap. If you toss out standard compatibility, or want to make a really non-standard format, then you can make the dimensions whatever you like if the camera can handle it.
Leaving a large dark slide partially inserted risks damage, or having a breeze displace the holder. I do it with roll-film holders which are designed to work that way, but do not with 4x5 and up.