Ok, how simple do you want?
If the boat is balanced on certain points of sail, then a rubber from an inner tube may suffice on the tiller. In the old days, downwind, twin headsails sheets (or 3rd sheet) on the tiller.
Good luck. Oh -BTW I have Fleming for sale....
If this were the case, then there would be no need for a self-steering system across the board, big or small boat, offshore/inshore. The point is that self-steering traditionally is there to accompany the >26 ot <60 offshore.
However, its function could be useful for a wider range of size vessel and in a wider range of location. (specifically inshore) there are a LOT of 20-30 foot vessels around me, hence the thought, and questions. I am speaking of above-the-water systems (ie, paddle/vane to steering systems).
I am not aware of too many <30 vessels which are self steering via sails alone. The lack of effective keel/draft leave this whole range of boats out of luck with steering systems. More precisely, I am not aware of that many skippers. We aren't all gods of the wind and sea.
A rubber band will only keep a perfect sailplan on limited points of sail, and it will drive you in a straight line even when the wind changes direction past the radius of the rubber band destroying the system , whereas a vane driven steering system will continue to accommodate a large change in direction, more important too when discussing smaller boats in generally protected waters where wind shift is actually a much greater concern (remember, a self-steering system is designed to keep you on the wind, not on the compass...).
I suppose the discussion comes down to more of an ""effective turning radius". I am sure there is a name for that.