have you ever seen one of those clamps that are used
to hold gobos infront of lights, or reflective / black/white &c cards
over. below to the side &c of lights ?
they are clamps on both sides and an adjustable snake/arm in between ...
can you just get a safelight filter, from an old bullet or square safelight
and just clamp one side to the filter,and the other to your enlarger body ?
you won't need to cut or trim or anything, ... and it would be adjustable ...
and probably would cost only a few dollars ..