Since the raised platform proved itself to be reasonably stable, the switch idea came back in favor.  If the switch was on a flexible arm at center roll level, it wouldn't have gimbal lock.  It wouldn't reduce the build area height.  The deflection would only be the cross section of the roll.  It's a lot simpler than string.

The leading idea was a photo interrupter like the prusa to reduce friction.  The leading phototransistor goes from 1k on to 100k off.  They're obviously designed to use 10k resistors.

The mane problem with fabricating fork structures is the supports.

But there is a way to cut them out by melting them in half.

Then cutting them out with the xacto.

The latest fork structure was on the way to reaching the full range.

But there was still a missing degree of freedom.  The photo interrupter would have to be a flat disc which detected interruption anywhere in the disc.  There are cameras which can probably do the job.  The string idea still had the lead in simplicity.