[Woodcarver] Need adviced on relief carving panel construction

[Alex Bisso]

    At one time I carved a sign - sort of a cut out relief - of an elevated lighthouse.  I did not  but any initial cambre into the the pieces of basswood that were glued up for the piece (the center piece with the lighthouse and the two wings - see photos named Summit Lighthouse and Summit Lighthouse Painted  in carving album at http://www.picturetrail.com/gallery/view?username=abisso&x=19&y=15 ) and the wings have badly warped outwardly.  I also did not seal the back which I think also contributed to the bad warping.
    Now I plan to recarve the sign, this time as a complete relief carved panel that will be approximately 24" wide and 26" tall and I have conflicting thoughts about how to glue up the panels.  Although I would like to be able to use the original wood carving for the outdoor sign, I am thinking that even with building in cambre, sealing, etc. there will still be some warping and weathering of the wood (sign is exposed to harsh weather and afternoon/evening sun).  Therefore I am thinking about keeping the carved sign for inside use and doing a mold and cast of it for the outdoor sign.  Here are my questions:
1.  If gluing up the panel with the boards running the long way, how many boards/joints are recommended and what degree of cambre should be put it each?
2.  If I build in the cambre, will this make doing a mold and casting more difficult because the panel might not be flat upon completion? 
    Discussion:  My original sign was nice and flat after carving and only warped after exposure to the elements.  It would have been easy to make a cast of it.  Since I have almost no experience with relief-carved panels, I am concerned that after initial carving, a panel with cambre might arc up some in the middle causing some difficult in the molding and casting.  
If you have any experience with doing something like this I would like to consider any advice that you might have to offer.  Thanks.
ALEX
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