Something else to try. I use an old piece of aluminum wing joiner tube for bigger stuff. Trick with it is not to put teeth on it. Sharpen the inside bore with a drill bit.
Makes a nearly razor sharp edge and will slice through foam like the proverbial hot knife through butter.

For smaller holes, a piece of CF tubing, same sharpening of the inside bore, but use a razor knife instead.


On the smaller tube I can do it by hand, chuck it in a hand drill, or I've even used a sharpened CF tube in a drill press to install the rods in foam birds for strength.
Use the drill on slowest speed, and walk the bird into the rod, get the length needed installed and trim off the excess.

I can do a 24 inch long 3/8" CF tube in about 5 seconds. Even got good enough at it I can run a 1/32" CF rod in 1/16" foam for strength in micro birds.

Oh one other thing with the sharpened aluminum tubes. Heat em up a bit and they make a perfect circle in covering. No ragged edges, covering stays tight, and no corners to tear.