Did you pre-wash? That will often "set" the fabric grain. I have found that panels are often off-grain and when I want to use them, I quilt the panel first, square it up, and then add borders and backing [in a quilt sandwhich], one side at a time, and quilt each one before I add the next side. It is really frustrating to do all that work and have it "buckle". My guess is that it was not your measuring OR sewing, but the original panel that was off.
It is a nice thing you are doing, for a special gift...men are harder to find the "right thing to do" for them.