To tighten the hot wire, drill a hole in the base of one of the pipes. Stick a screw driver in the hole and use it to turn the pipe to tighten the wire. I found that using vise grips would eventually crimp the pipe, causing it to buckle under load.
I used hose clamps to secure the hotwire to the pipes. I wrapped the wire around the pipe, then held it in place with the hose clamps. I found that the wire breaks more often if threaded through a hole in the pipe like shown in the plans.
Always loosen the wire BEFORE turning off the power supply. A very hot wire tends to expand and yield. That's why we tighten the heck out of it before hotwiring a foam core. However, the wire will shrink up when it goes cold! Not loosening will fatigue the wire prematurely, causing it to break more often.
When hotwiring, lean the saw into the direction of travel at about a 45 degree angle. In this way, the wire is being pulled along, not pushed. The wire will tend to ride over bumps and won't snag as often on imperfections on the hotwire templates.
I found it helpful to use metal yardsticks for making the numerous straight cuts to size the foam blocks to proper dimensions before cutting the wing cores. They have numbers on them already (duh!), so what a great way to call out numbers to synchronize the cuts between two people. I drilled 1/8th inch holes in the metal yardsticks every inch or two and use drywall screws to hold the yardsticks in place. The holes are just the right size for the screws.
Mike Skorija (aka Dust) does the same thing with carpenter's squares. He gets a square cut every time. That sure beats marking the foam, then setting the straight edges in place. One step instead of two. John McAvoy (aka Waiter) took that a step farther and suggested using the 4-foot T-squares and longer straight edges used for squaring up and cutting drywall.