I gotta say, this method just makes too much sense to me. I'm surprised it's not the defacto way to do it. I've watched John videos on his AGM-33 Pike and I cannot imagine what a messy pita it was to fillet those long fins. With my upcoming 1/2 Scale Phoenix build I'm trying to learn every handy construction tip I can. And I'm learning a ton from Rich and John!
Here's how I'm going to make my fin pockets, at least for my 4" Madcow Phoenix. Simple 1/2" foam cut and hot glued to the MMT. Once the MMT is inserted in the body (sans fins) and affixed, I'll pour or inject some epoxy through the fin slot in the body. It'll be captured by my foam fin pockets and I'll lower the fin in place and wait for it to dry. Zero support is provided by the foam, but it doesn't need to provide any. The strength of the bond will come from where the epoxy contacts the MMT and the fin, just like a regular fillet does but thicker, and heavier now that I think about it.
View attachment 280330
View attachment 280331