One of the hardware stores I go to gives away wooden yardsticks. Every other time I go there I pick one up. I cut them as I need to for 12, 18, and 24 in. rulers. When I go to mechanically align surfaces, I take 2 of whatever size it takes, and either 2 spring loaded clothes pins or a couple of the little spring clamps from Harbor Freight and put them along the surface, one at the top, one on the bottom. That gives me the best mechanical alignment I can achieve. Now when I make the rod and adjust the clevis for no pressure back or forth when I slide the pin in, I know that the surface remains as centered as possible, and trim inputs should be pretty minor.