1. doing box lining by hand is a lot harder than you might suppose.
2. on the couple of occasions I've done something on a frame repaint that required long straight lines, I just bought vinyl decals and put them on at the same time as the other decals. Then I just cleared over everything with urethane 2 stage paint.
Do a couple of searches on Amazon for "pinstriping decals" if you are interested in this approach. ..
.here's one result