I usta take my back seat out to fit my snowboard in the trunk/cabin. Just remove the back and leave it in your garage or whatever. then put your board under the convertible top boot and into the inside of the car.
Works for me... Yet I don't do it anymore now that I have a 2nd car, the ****mobile!
Welcome to the impracticality of owning a convertible. I've been able to fit a number of large awkward items with the top down, but with the top up you're pretty much SOL, unless you follow the above advice and take the rear seat out. I can't really envision that working too well either but I guess you have to try it to find out.