Some people leave the D-box black and others prefer to make it reflective, both to prevent heat build-up on a sunny day and for aesthetic reasons. I've tried both and prefer to make the D-box reflective. I add 0.5 micron silvered mylar to the complete D-box just before covering the wing. I adhere it with 3M Spraymount, working from the top rear of the D-box round the LE and back across the underside. Its quite easy to do provided you go easy on the Spraymount. Provided you haven't rubbed the mylar down hard or stretched it, it is easily repositioned to remove wrinkles. The mylar added around 1.3 g per wing half. Doping the Salzer tissue on is what really secures the mylar, so you only need enough Spraymount to keep it in place during covering.
My latest wing was covered with Ikarex 31, a polyester kite fabric. Technically it is a plastic film with a textured finish. It weighs 38 gsm. At this weight it is OK for F1A and built-up F1C wings but is probably too heavy for other uses. Ikarex comes in a lot of nice, bright colours and is extremely puncture resistant. It is easy to apply. I adhered it with Evostick (a British contact adhesive) thinned to the consistency of thin dope with hexane (Evostick sell it as thinners) and then shrunk it with a heat gun. If you're tempted to try kite fabrics be sure to use only Ikarex 31, the new polycarbamate coated Ikarex or Toray because many of the other kite fabrics are not polyester and will not heat shrink.