Still not getting why you are opposed to through the wall. At the weight and power you are talking about, you will need all the help you can get. Losing a fin on a heavy rocket is just asking for trouble.
There are situations where surface mount is a must. When done correctly there is no problem with strength. WHEN DONE CORRECTLY.
The rocket above flew 3 times in different configurations weighing from 300-375lbs.
Cutting slots in a tube this size would compromise the tube strength & is not needed for any reason. You want the strongest tube possible and with several layers of carbon Tip-Tip,,,,,,the fins ain't going anywhere.
You must figure what stresses are involved and design around them.
When you build rockets this large.....it IS rocket science! You better have weight and thrust profiles figured correctly or your project will fail disastrously.
If you have never been part of a project like this, I can see why the skepticism. In our case the P-motor was bell shaped 9in at the nozzle and 10.5 at the top. No motor tube can be used.
The motor forward end had a ring receiver with 6 threaded holes. There was an aluminum 1/4in BP in the airframe. The bolts go through the BP and into the ring on motor to hold in place.
Here are the pics of minimum fincan; Looking down see the Aluminum CR with motor bolted on [through center]
There is a wood ring around motor on nozzle end NOT a CR..it's just glued on motor to prevent base drag pulling flame and heat up into fincan around the hanging motor.
Motor in ship box see how wider on top.
Top of motor [carbon tow wrapped around the grain, no metal case] showing built in metal ring to mount.]
There are times you need all the open space for other reasons.
You may do several motor mounts that fit into the 12in airframe. [removable]
6M's around a N.
6in mount surrounded by 54's
4 cluster M's
8in mount. [P & Q}
You can make these internal mounts interchangeable of different flight profiles.
all that being said...it is a rarity to see a bird this size A: survive intact without damage and fly again. B designed for multiple flights.
Big Rockets with huge motors are usually designed around that motor and looked upon to be one time use.