A few years ago, I retrofitted our entire fleet of finished model rockets with ventholes to accommodate a JL Altimeter 2. I developed a process for making altimeter ventholes which works pretty well.
First step is to decide whether to use 3 or 4 equally spaced ventholes. The answer for each rocket depends on whether the number of fins on the rocket is 4, or a multiple of 3.
Next, I extend the fin lines to the nose-end of the body tube using an appropriate sized piece of angle iron, or 90 degree wood moulding. I mark the fin lines lightly in pencil on the OD of the finished body tube (lightly enough that they won’t mar the finish, but can be wiped-off later…Sometimes I wrap the body tube OD with blue masking tape before marking the lines, if I am confident the clearcoat won’t lift off). I usually try to place the holes mid-way between the fin-lines to avoid the potential for aerodynamic interference from the fins. (However for practical purposes the fins are usually so far downstream of the venthole locations that putting them between the fin lines really isn’t necessary.)
Locating the holes axially along the body tube is a bit of an art. I typically attach my Altimeter 2 using the supplied swivel connector, either directly to the nosecone eyebolt, or to a split ring attached to the eyehole of a plastic nosecone. I normally locate the holes approximately at the position where the altimeter will hang during flight. When locating the holes, it is desirable to have them 1-2 body tube diameters downstream of the nosecone-body tube joint, or away from any decal edges, to avoid aerodynamic distortion of the pressure at the ventholes. It is necessary to have the ventholes far enough down the tube to avoid being covered-up by the nosecone shoulder (Duh!). It is also necessary to avoid having the holes at a location where they might be blocked by a stowed parachute or streamer. You want to avoid putting the holes at locations where they will line-up with the seams of a cardboard body tube (so you don’t weaken or split the tube). Finally, it is necessary to have the holes at a location where you can reach them with the tip of your finger inside the tube. (More on why this is necessary later).
Once intended hole locations are selected, I lightly mark them in pencil, check that all of the above criteria are met, then start making the holes.
To make the holes, I start with a very sharp sewing needle, and gently but firmly push the needle thru the body tube from outside to inside. (Using a thimble to push reduces fingertip distress.) After the first push thru, I use a piece of sandpaper and my fingertip to knock down the cardboard volcano on the ID of the tube, to remove anything that could snag the recovery system. (I find using a fingertip better than trying to do it with sandpaper wrapped around a dowel...So finger length often ends up being the deciding factor in determining how far down the tube the holes end-up.) After sanding, I then push thru the needle again, sand again, push thru again, etc. repeating a few more times until the holes are clean. I then move on to gradually enlarge the holes using a succession of slightly larger items…straight pins, small nails, larger nails…using the same push thru/sand/push thru/sand process, until the desired size is achieved. (A micrometer comes in handy to measure the various pins and nails used to make sure you get close to your desired hole size.) After the final sizing step, I take a conical bit from my Dremel tool set, and BY HAND, use it to break any burrs on the outside surface of the holes, by centering the tip of the bit in each hole, and slowly working the bit back and forth by hand. At the end of this process, I have a set of nice clean holes of the desired size, with no damage to the finish on the exterior of the rocket.
Notice that all of this is done by hand, without using any power tools. I found this to be a prudent precaution. My experience has shown that using power tools on delicate work with model rockets often leads to trouble as bad things can happen fast. Hand making the holes makes for more precise positioning, and avoids inadvertent damage to the finish due to slips.
After retroactively making altimeter ventholes for all of our legacy rockets, I made it a point going forward to put altimeter ventholes into every new rocket during the initial build, before finishing. When making ventholes on new builds, I use a similar process, but take the added step of applying CA to the selected hole locations (locally saturating the tube) before making the holes. In addition to strengthening the tube, I find this results in neater holes with fewer fuzzy burrs. (I didn’t use CA on the retrofits…just carefully pushed thru the existing finish, gradually sized the holes, and sanded the tube ID between steps.)
This method has worked well for me so far on many rockets with cardboard tubes, up to mid-power rockets. Give it a try.