The Estes Altimeter instructions are vague about how big the pressure sampling holes should be.
They show small square holes, very hard to cut with an X-Acto knife.
Perfect Flight makes small "flashing count" altimeters for $25.00 CLICK HERE
From the Firefly PDF instructions here's some more in depth information:
Static Pressure Sampling Holes
You must drill one or more clean-edged holes in the payload
compartment to allow outside air pressure to be sampled by
the altimeter (see table below for recommended sizes). These holes should be as far away from the nosecone shoulder and other body tube irregularities as possible (at least 3 times the body tube diameter or more) to minimize pressure disturbances being created by turbulent airflow over the body
tube. Sand the area around the hole as necessary to eliminate flashing or raised edges.
Best performance and greatest accuracy will be achieved by using four smaller holes distributed at 90 degree intervals around the body tube’s circumference instead of a single larger hole. When using four holes, each hole should be ½ the size of a single hole as noted in the table. This will minimize the pressure variations due to wind currents perpendicular to the rocket’s direction of travel.
According to the list above, the holes I've been punching in payload tubes have been WAY too large!
These are directions for the Perfect Flight Altimeter, not the Estes Altimeter. There might be a difference in the recommended hole sizes and different brand altimeters - who knows!