Mike

The tonneau keeps passenger seat dry & you keep drivers seat dry. You will likely find the foot wells will be wet so take care what you put there. If its dry the passenger seat & foot wells will have a coating of dust.
To mitigate ingress of water & dust you may want to silicone seal between the frame & floor & tape up the large gaps behind the seats with aluminium tape. Have a look at this thread discussing sound proofing.
http://www.talkmorgan.com/ubbthreads.php/topics/180047/1
No doubt you will be grinning ear to ear whatever the weather on Thursday.

Tim