projection lens with no stops and film ...
how about not having a ton of light
and a longish exposure ... focus on a box
or something in your seat before hand ( that is what i do )
and if the exposure is long enough it won't even know you sat down
and got up
speaking of string, another option is to have something on a string to drop in front of your lens.
release the string when the time is up. ... kind of like a manual shutter outside your lens ...