I'm in the process of building the annular grid fin for my previously un-named rocket, now called the Deanston Marauder, shown below.
The grid is cut and is being attached to a 12" sewing ring. It will sit at the base of the 3+ inch body. I have no idea if it will be stable. Additional design mods may be coming.
I'm also in the process of building a 29mm-powered backsliding super-roc from a 8' plastic florescent tube protector (a.k.a. Chasin' Tail). This is almost exactly the diameter of a BT-60. I found a fireworks nose cone in the junk drawer that fits perfectly over the end-cap. On the rear end, I'll have a 16" mailing tube with a 29mm mount. The cardboard tube will provide a secure mounting point for the motor mount, fins and rail buttons. It fits loosely in the plastic tube and will have a tape thrust ring and will be friction fit. This works for motors, so why not the whole fin can? Being a backslider, there will be no ejection shock to worry about. Finally, as I've been studying the various backslider articles on the web (Pete and Robert Alway and Mark Fisher). It actually looks like the added weight of the mailing tube will also help keep the CG between the Barrowman Center of Pressure (BCP) and Center of Lateral Area (CLA). Photo's to come.