If the exposure is very long and you are otherwise efficient about it, a hat or a small black box or a loose-fitting lenscap is quite manageable. The light leaks are not really that much of an issue.

I've even done portraits with open flash technique this way (pull darkslide, remove cap, fire strobes, replace cap, replace darkslide, lather, rinse, repeat).

Other possibilities are packard shutters, front-mounted shutters like the "Luc" shutter, and Weston used something called the "Worshing Counting Lens Cap" or something like that, which was designed for this purpose.