FWIW, I've done my L1, L2, and L3 on increasingly large (and now fiberglass) tube fin rockets. I also started out working on it with my son, who has since graduated to things that are far more complicated than what I build. (The attached photo is a 3 inch rocket with 3 inch tube fins, and the launch video is here:
https://vimeo.com/317747037)
A few comments/pieces of advice I've learned:
1) I found it very useful to make a spreadsheet with all the items going into the rocket weighed and sized. That way you know where the weight is coming from after assembly.
2) At the top of the motor mount, I installed an eye bolt that I threaded a loop of Kevlar through and just leave in place. Everything attaches to that and it makes later customization very easy.
3) I initially flew this as a single deploy with a JollyLogic chute release. It has since been edited to support dual deploy with a different front end. Don't forget a vent hole in the body to where the chute release lives.
4) I glued the tube fins into pairs, and then glued each pair onto the rocket. (When the body tube and fins are the same size, I do two opposite fins and then add pairs, but since these are different sizes, that won't work.). It is much easier to keep everything straight and you can work more quickly if you can do them off the rocket.
5) Think about launch lugs carefully. Because the tubes are reasonably small, you likely have to offset the lugs a bit from the body so the rail doesn't hit the tube. I 3D printed them with a slight rise (1010 guides), but you can also just use a nylon spacer.
6) I built a thrust plate of plywood. Because there aren't through the wall fins, this guarantees that the thrust is transferred to the body tube and minimizes stress on the motor mount. This thing has since gotten beaten up a fair bit (including become the stage 1 of a 2 stage tube fin) and nothing ever breaks.
