Internal frame is done on numberous commercial rockets IIRC. Something to think about though - what load does it have to take? Under boost, the rocket weighs what? Going off under how many G's of acceleration (plus 1 for earth's gravity)? So what sort of compressive force is this? Now suppose it hits wind shear? How much side load at max-Q does it have to take? What are the aerodynamic loads?
A tube is a very simple and pretty robust structural member.
You're looking at L3. That's an M or larger. Probable thrust, say, 300 pounds to over a thousand? Mass fraction of motor hardware and propellant, probably around 30% give or take a lot? So, the structure would then need to take roughly 270 to a thousand pounds of compressive force applied at the retainer if rear retainer. Or a mix of compressive and tension if applied at the head-end of the motor which is better structurally for the sorts of motors we use as the compressive force is greatly reduced by being split up and most structures are stronger under tension than compression. That's with no margin of error. What sort of margin do you want? 100%? So double the values.
Note, 50% margins are considered adequate for many parts of commercial rockets; however, they are much better engineered, tested, certified, and operated by crews of professionals. At least, that is my understanding
So, picture most of a ton of weight sitting on your structure, with the structure supported at the motor thrust plate wherever that is. Now push on it some from various angles, yank on the fins, etc. Will it take it? If so, please, go ahead and build it! It'll be a really cool project!
How big a motor do you want to eventually put in the project? I warn you, once you fly an M, you'll want to fly an N...
It's also a really cool project for a second rocket after getting the L3. I did my big complex project for my L3... I'd say it delayed the L3 cert flight by a year over what it could have been. More, if design time were counted! Many L3 projects are built as simply, as basically, and as bullet-proof as possible. It's purpose is to get the cert in as close to a guaranteed fashion as possible in rocketry. Afterwards, it is good to go for lots of cool flights while building other projects.
Good luck whatever you decide!
Gerald
PS - I never mentioned recovery loads.