Hello John,
The SLEC kit for Sea Urchin is an updated version of the original Aerokits design distributed by KeilKraft, not sure who or where Roadrunners review was ? but the original design had the deck fitted after the hull sides and trimmed flush with a 1/8 square rubbing strake externally over the joint, did you have a drawing with your kit ??
I'm not sure you need a "sub deck"
Hope that helps 😀
Regards Ray
Edited By Ray Wood 2 on 02/05/2020 16:17:48