Over the years, there have been a few different kits available to do this. The fiberglass kit was something different, but Paul at Flyleds put together an outline for the aluminum coves that Van's used to supply (but doesn't anymore).
You can find the rough plans here... ask me nicely enough and I might be convinced to CNC a version of these for you all.
It's also worth noting that Flyleds has a great position and strobe lighting kit that will fit these, and we carry the lenses you will need on our site (NOTE: the lenses will only fit if you have the aluminum coves mentioned earlier in this post. If you've got the fiberglass ones or the rare kit with the PAR36 landing light in the cove, our lenses will not fit)
Find those items on our site at the links below, or get the kit directly from Flyleds if you prefer.
I copied those pics off a google search from someone's online build log several years ago. So far, the only lead I've gotten is someone supposedly had the molds as was making them for Van's years ago. But, there were no more available, and I was never able to get in touch with the guy who had the molds.