Just as I thought, same as one I have.
I have 2 options for you:
1. For Shear-Pins....Using 1/2 or 3/4 in. wide tape, wrap several layers in front of rear ridge, butting the tape up to ridge. Same for front ridge, depending on tape width, either in front or back of ridge. You will be a bit higher than ridge, as you are making a "seal". Not to tight a fit, just snug.
As mentioned using thin metal flashing imbedded, makes for clean cutting of pins, rather than tearing out your holes. Being cheap and lazy instead of going to Lowes...Hd..etc & looking for thin shim stock,[you may have some?] I just cut small strips from aluminum soda can & after my first wrap of tape, placed one/2/3, on tape & continue wrapping, so metal is in the tape layers. When finished you install into payload and drill pin holes through everything. Simple, neat, clean, easy.
2. I don't do that anymore. I use shear tape on cardboard rockets. After a number of flight, eventually your holes in airframe will get loose & must be fixed or rotate the NC and drill new ones. A 1/2 in. wide strip of masking tape has same holding power as a 2-256 pin. [as used my me anyhow]
On this 4in I used 2....not on the cone, LOL but to hold shoulder into airframe. I had a motor sooo long, it prevented the cone from seating. It raised eyebrows at the RSO table, but easily passed the "shake" test.
View attachment 297674
Do your wraps around shoulder of nc to seal & snug fit & just 2 strips to hold cone on. Your cone is very light weight, so I doubt 3 pins or tape is needed. Not very pretty but works great & keeps the integrity of airframe intact.
So those are my 2 tips. There are many, many more ways to incorporate shear pins into cardboard, I'm sure some will pop in soon, on your thread.
Edit: the absolute best way, in my opinion, to prevent damage to airframe is to laminate a 1-2in wide strip of fiberglass around the top of airframe. More work...better results.