I've built my Cirrus out of the 5mm Luan which is sold as underlayment. I searched through a couple of stacks and found plenty of panels with uniform color and (more importantly) no voids. For my first boat I reasoned that the cost per sheet ($8.00 vs $39.00) was worth it. While I was shopping more than one participant in these forums thought the luan was a decent alternative to okoume.

In the end if you make a mistake lofting or cutting tossing a $8.00 sheet doesn't feel quite so bad as a $40.00 sheet