I'm scratch building one & am going to make them detachable. The Macchi has a narrow undercarriage track so I can see the tip tanks getting scraped all the time so I figure they would be easier to repair & paint if they can be removed.

I have built my wings with an extra rib bay & a 3mm ply rib at the tip. The idea was to slide the tank over the end of the wing & screw into the ply rib which would be under the outer surface of the tank.

Originally I was going to fit 2 small air tanks in the wings rather than one large one in the fuselage as space is a bit tight. I cannot get these to fit where I was going to fit them. I'm now looking to fit them in the tip tanks but I can see that being a problem with how I was going to slide the tanks over the tips. It's a work in progress I guess. - John.