This is one good way I used as well back in the day of classic builds. I've built the saw from various things, like cans, even schedule 10 pvc pipe. I even lined the foam with foam coffee cups in lieu of balsa. Worked well

Last time I built a retract model about 10 years ago, I tried something a little different. ACP sells a nifty little tool called "Hot Hand" or something like that. It's a 5" long piece of stailess tubing about 1mm in diameter, on a handle with power leads. Connected to a 12 V 1 amp power source such as some transformers one may have lying around, this thing is excellent for carving foam.

Anyway, I cut a pair of templates from card stock with the retract holes (including the strut section) located where I wanted. Cutting perfect circles in the card stock was simple....fashioned a simple hole cutting tool using an Xacto blade, a popsickle stick and a push-pin. Both templates cut together to assure precision. One template taped on top of the foam wing the other taped to the bottom, lining up LEs TEs and root chords. The Hot Hand tool cut the holes perfectly. The same templates used on both wing panels.

FWIW2U