You are apparently setting up your own work. As you gain experience you will learn to expect distortion and allow for it. This is very valuable. You should accumulate a few tools to allow you to set things up in a repeatable way. A carpenters square, a fair sized straight edge, a measuring tape, and a chalk line are a start. A few blocks of wood and some wooden wedges, perhaps some clamps, but I know you have a small budget, to hold things in position. That and a good working brain, which you already have. Set up then one tack and recheck, remember how it moved and readjust, then another tack and so forth. Use this as a learning experience, it won't last forever and you can learn a lot. If you have to make a lot of the same thing think about building a jig to set the parts up in (also good experience). Work smart, it's easier.
Bill