neurotex,
i've found that sanding blocks work well, or just a single piece of sand paper even. using just your hands to sand it will let you have a lot more control over the pressure and heat. its very time consuming to sand stuff like that, when i did the prairie duster, i spent more time sanding balsa than i did assembling the plane! it pays off in the end though, the first one i built i did no sanding on and the empty weight was 2 grams, the 2nd one i did, i sanded most the balsa as much as i could, and even though the 2nd one had the extra weight of tissue and adhesive, it still came out at only 1.7 grams.

maybe some 1/16th inch balsa would work for the bottom half of the channel to give you the streangth while keeping the weight low. then you can use foam or whatever else for the upper airfoil. do you have a picture of the ribs you plan to use for the channels airfoil? i retraced the one that was drawn in the 3 view, but looking at the pictures of the twin channel wing, it doesnt look the same. i'm wondering what the changes that were made from the Langley test plane compared to the twin channel version.

i think this project deserves a thread of its own, its got some serious promise!

nick