Acme rail guilds unless you really are dedicated to rods. I personally don't like rod whip on larger birds like this. Use as large a rod you have (obviously 1/2" is overkill) and a thin wall brass or alaly tube trimmed down. You can nest the tube if the slot has already had the die-cut removed then add fillets to fair them in. I've built a half dozen of these and the last I glassed after taping the inside with adhesive tape and adding a little thick CA over the outline on the outside and sanding it flush. Don't expect them to live well above mach using the plastic fins. You can replace those with G10 the same thickness as the 'fin-lock' ring gaps and fillet them in. Please don't ask why I waste my time like this- I don't know either!