I've long felt that the window net section of the SCCA General Competition rules was very weak, not giving a good description of what is needed in a window net and mount.
Window nets are required in all closed cars in the SCCA, with one exception, which are the closed cockpit sports racers. The only example of such a car that I've seen is the Diasio replica of the Porsche 962; in cars like this you can use arm restraints instead (window net mounting in this car is difficult, so it's a reasonable allowance.)
For the rest, there are some basic principles to consider. First window nets should mount to the roll cage, not to the door. I don't know what IMSA rules look like now, but we used to occasionally have discussions about this with people bringing IMSA firehawk cars over that had door mounted window nets. If the net is on the cage, and the door pops open, you will appreciate having the net stay in place.
Window nets are supposed to keep your arm inside in a heavy hit. You therefore need to consider the strength of the net system when your arm hits it HARD, not when you push on it lightly. I've seen people using elastic cord, and people using hollow aluminum tubing through the top and bottom. Neither approach really cuts it; use steel rod the length of the top and bottom.
Do not make holes in the window net webbing. SFI certified window nets will, to the best of my knowledge, cease to be SFI certified window nets when destructive modifications are made to them, so making a row of holes for a "shower curtain ring" mount is right out (yes, I have seen these types of mounts presented in tech. This is a "you don't race 'til it's fixed" kind of problem.)
Typically, one edge of the net is quasi-permanently mounted and one has a release. I suggest that the top have the release; egress when upside down is rarer than egress when right side up, so having the net fall down when released makes things easier when you're in a hurry.