For one of the BT40-based rockes in the 1964 catalog's Launchpad Special, I built the Dirty Bird III. It has the right fins, is about the right length, but has a different nose. Close enough, I figured.
The problem with building the Dirty Bird III is that it was designed with a plastic fin canister that isn't available any more, even when I looked on eBay. So, I built a model of the canister to fit my model of the model rocket.
I started by making a thick wrap of Kraft paper around the rocket, gluing it as I wrapped. When it was the right thickness, I let the glue dry. (I found the dimensions of the fin canister in
this thread.) I built the fins originally from 2-layers of 3/32" basswood, thinking I'd sand the taper into them. I gave up on that, and remade them with 2 layers of 1/32" plywood, with a shim between them to make the taper. I filled the leading and trailing edges with body filler that wouldn't shrink. Some careful sculpting of the ring, and epoxy to hold the fins, and beveling on my belt sander, made a pretty good replica of the fins.
Then, primer, paint, and decal.
Still no 3-fold mount for the shock cord. Nor an engine mount. The fin canister comes off, you put in the engine, and put the canister back on, making sure it doesn't come off. The instructions called for a hollow wooden thrust ring, but I made mine from BT-30 and BT20 tube, and a centering ring. BT-30 fits exactly inside of the BT40 (which I hand rolled), and the BT20-bit inside the BT30 once I trimmed out a section.