Cardboard body tubes are super easy to work with.  I used an exacto knife to lengthen the fin slots and remove the rear of the slot.  I'll be building the MMT and fin section outside the rocket and then sliding in as one assembly.

Here's a dry fit of the assembly in the body tube.  The extra holes in the rear centering ring are for 1/4" brake conduit.  That will allow me to run igniters from the outboard 29mm motors to the electronics bay in the middle of the rocket, for use in airstarts.  Not added yet are additional holes needed to mount 1/4" flange nuts for motor retention.