Oh yeah -- I wuz going to post a photo of our El Cheapo tiller lock. Here it is: simply a notched piece of plastic loosely screwed at the front and tightened by a knob at the rear. Thru the V notch runs a grippy length of 1/4" line that is tied to the stern cleats. Ta da.
Works amazingly well.
Oversheeting the genoa: it would tend to increase the sideforce on the sail, pushing the nose off the wind. So yes, it should counteract weather helm. Next time out, try the converse relationship, keeping the genny at ideal trim while undersheeting the main slightly. We often sail the Bucc18 with the mainsail luff 'soft' for several feet. This is SOP on that boat, which has wicked weather helm & few depowering options. It's also a way to stabilize the boat when you need to go forward: the basis of heaving to is an oversheeted (generally backed) headsail and an undersheeted main.
The comment about keeping the slot open in heavy air is a good one, but not especially germaine to your helm issues. Cheers!