I always leave in a little dihedral, I'm not an aircraft engineer but the next kit I build like an Extra or Kaos that has no dihedral will be the first so I figure if they have it then who am I to remove it. Even if I lay one side flat and add a block of about 1/2 inch to the other that adds a little. I block both tips though so you do the math.
I have a sheet of 1/32 door skin I use for the LE and turtle decks on the 4*s but also use contest grade 1/16 balsa. Both are light weight but the sheeting is thinner.
Do a search here on RCU, Ed Moorman has posted photos of one of his that he bashed to look like a warbird, perhaps the coolest one I have seen.