Altitude record attempt using the O3400 in minimum diameter style.

[ColumbiaNX01]

First, I agree with Justin about going with the manufacturer of your tubing.

That said, considering this rocket is going to be hauling arse I would go for a cone with a large l/d. 5:1 or more. VK or conical

The one I have is a filament wound 5:1 Conical profusion that I bought when I bought the rest of the rocket last year.