I just ordered some Bulnesia sarmientoi to use as wood slats on the rack for my Fargo, and will have some extra if anyone else wants some. I will be having it milled to thickness to get a few extra slats out of it. The Bulnesia sarmientoi be about 4 inches wide, and should have enough to do somewhere between 4 to 9 "grocery bag/12-pack" sized racks.
Bulnesia sarmientoi is the second densest wood in the world (between .92 and 1.1 gram per cubic centimeter), and has a natural wax in it which serves as a finish when rubbed. It also goes from a golden yellow to a fabulous green and blue streaking when left in the sun. It is so hard, that you should be able to tap it for M5 x .8, and bolt it on from the underside of a rack for a totally flat top surface.
/sales pitch
I'll only ask that you help defray my costs. It looks like it'll cost me about $60 to get it and have it milled, so worst case, about $15 for a rack's worth.
Let me know if you are game.
Also, Lee is going to be getting some old street signs for the same intended use, so bother him if you want in on sinage.