That will be some odd cuts there bro-hymn. if you feel like dumping out twice the cost practically for a 4x10 or 5x10 sheet (can't remember) you're gonna have some ugly seems at the top or bottom of whatever you do.
As for the panels, I would probably go with alumacore probably to add some weight, I would also support the top of that sign with flight cable attached to the frame. IMO he should have figured out what he was going to do before he made the frame ... at this point it's trying to get something to work, not look good.
http://www.flickr.com/photos/aaronbstitt/4132330359/
Honestly, he should start over with something like the link. It will be more sturdy in heavy wind and weather and he can then make it to be something functional like an 8x20 with the supports at 4' sections so if something DOES happen to a panel, it can be replaced easily without gimpy added cuts to make it the full size.
-edit- oh and if he does go with the setup like in the link, there are companies that make LED work lights that dump out 7500 lumens and use less than 6 or 10 amps each on a 12v source ... perfect for trailer use so it doesn't kill the vehicles alternator since they use 1/10th the juice.