Have been considering some sort of tiller control for single handing.
This is in addition to an auto-pilot as often the AP isnt connected
and all you want to do is go forward for a few sec to clear a line or
something. A back issue of Practical Sailor had a review of several
tiller control systems such as the "Tillerlok", or "TillerTamer" and
others designed to keep teh tiller amidships. These all involved
something costly, complex, or cutting a hole in the boat for mounting.
Most of them held the tiller rigidly amisships which I dont think is
desired.
What I propose is the following:

Obtain a 4-5" piece of shock cord. In its center, put a loopp to fit
tightly over the tiller so that it wont go very far down the tiller
without some force. On either side of the tiller, use a piece of 5/16
line to tie a Prusik knot around the shock cord (prusik knot is used
for mountain climbing and vertical caving. It wont slide down under
load unless "unloaded" by hand but can be easily re-positioned). The
other end of the 5/16 line has carabiners for attaching to your
slotted toerail or other place on either side aft of the loop around
the tiller.

When KNOT in use, one carabiner is taken off the toerail the central
loop is off the tiller and it is stowed to one side.

To use:
Slip the central loop over the tiller, stretch the other side with its
carabiner and put the carabiner on the toerail. With equal stretch on
either side, it keeps the tiller in the middle but allows you to
quickly shove it to either side against the pressure of the shock cord
if needed.

This also allows for some movement of the tiller instead of keeping it
solidly locked. It also allows you to adjust the pull on either side
by repositioning the prusik knot one either side to shorten the
effective shock cord length. This allows for a few moments of self
steering if you make sure you have a little weather helm and keep some
pull to windward on the tiller.