I use wire retainers with #6 screws and t-nuts for motor retention. The wire retainer is easy to make, and it only cost a few pennies per retainer. I make my wire retainers out of 17-gauge electric fence wire, but other gauges of steel wire could be used.

To make the wire retainer, first cut two pieces of wire. One wire should be long enough to form half of a loop around the motor nozzle, and connect from one screw to the other. The other wire is shorter, and only needs to form half of the loop plus some extra length to twist to the other wire. The length is not critical, and it's better to make the wires extra long initially.

While holding the wires next to each other tape them together using duct tape. This holds the wire together temporarily, but more importantly the duct tape will serve as a cushion to keep the jaws of the vise from nicking the wire. The spacing between the tape is determined by the size of the loop and the number of twists that will be put in the wire. The separation should be half the circumference of the loop, plus about 1 inch for two turns on the wire. For a 3/4 inch diameter loop, the circumference is PI*0.75 = 2.4 inches. Half of this plus one inch is 2.2 inches. For a 1.5-inch loop the separation would be 3.4 inches.

I place one end of the wire in a vise so that the tape is flush with the edge of the vise. I use vice-grips to hold the other end of the wire. The vise-grips should be clamped at the edge of the duct tape.

I spread the two pieces of wires apart and insert a rod between them. A socket from a socket wrench set can be used for this purpose. I use a socket that is slightly smaller than the size of the loop that I need.

While firmly holding the vice-grips I turn the rod 2 times or until most of the slack is taken up and the loop is almost round. Do not twist the wires too tightly or they will break or become weakened.

Remove the wire from the vise-grips and vise and cut off the excess wire on the short piece. The formed wire should look like the picture below.

The construction of the retainer is completed by placing it over a motor that has been inserted in the rocket, and wrapping the ends in a clockwise direction around the motor retention screws. The ends of the wire can be wrapped around the screws a second time to ensure that they will not come off when the ejection charge fires.

The wire retainer is very secure. It works for motors where the nozzle sticks out and for motors when the nozzle does not extend beyond the end of the motor. The loop should be made large enough so that it does not put stress on the nozzle, and the hot motor exhaust should not touch it. The pictures below show a 29 mm Road Runner motor in a 2.6 inch rocket and a 38 mm Cesaroni motor in a 4 inch rocket.