This year (2011) I'm trying out a Gray-B-Gon with an alternative drum design that might produce two to three times the evaporation rate. It turns out that the rate depends almost entirely on how much wet tulle is exposed to the air. How fast the propeller and drum turn is almost irrelevant.
A hi-perf GBG uses the same tray, with the power mast and idler mast mounted at the ends instead of the sides.The new drum is mounted lengthwise on the tray, and is longer: 39 inches instead of 26. (The long PVC axle is 31-1/2"). So there's a factor of almost 1.5. There's no room for the panty-hose bucket filter in the tray, so I mounted a 12" shelf on one corner for the bucket to rest on. The shelf is slanted inward, and cut away at one corner. The bucket bottom is drilled in only one small area, and that area is placed above the cutaway so it will drain directly into the tray.
The twine lacing, spiraling around the two wheel rims, actually creates two concentric cylindrical shapes -- the outer, where the twine zigs, and the inner, where it zags. Mount tulle on both the outer and the inner twine, and the area is almost doubled, or tripled when combined with the longer drum.
This can't quite double (or triple) the performance. The air isn't as dry, or free-flowing, in between the tulle layers, and the inner surface doesn't get as fully immersed in the tray. But it may be close. I plan to do some measurements once I'm on the playa.
Drum-building follows the usual steps, with some differences. Wet tulle attaches readily to nearby wet tulle, and surface tension keeps them glued together. So efforts are needed to keep the two layers apart.
Pre-2011 GBGs had no diagonal twine bracing. It's essential. The Construction Guide (on Instructables.com and this website) doesn't yet document diagonal bracing, but the Construction Steps Outline does.
On the standard drum, twine lacing moves forward two spokes per zig, or four spokes per full zig-zag (back to the first wheel). On the hi-perf, twine lacing moves forward only one spoke per full zig-zag. Fill every spoke space with a strand of twine.
It's easy to misdirect the lacing twine. As you lace, keep the twine moderately tensioned (not floppy.) A useful trick -- rather than pulling hundreds of feet of twine from the ball, mash the ball flat, put it in a small ziploc bag, and force it (as needed) between spokes. This will minimize loose but unlaced twine. Keep your work well away from the diagonal bracing, so you don't accidentally lace around a bracing strand. When you're done, or even before, pause to inspect: one zig in each spoke space,and all twine lengths resting on top of both rims, or underneath both rims.
On the standard drum, tension in the lacing doesn't matter; nor does tension in the tulle. On the hi-perf, laced twine must be snug, but not twanging tight.
The standard drum uses a half-width of tulle, 2-1/2 yds long. The hi-perf needs two panels of full-width (57") tulle. Trim the outer-surface tulle panel to 41" width (cut off 16" untensioned width.) Trim the inner-surface tulle panel to 35" width (remove 22".) Make a drum bulge from one of the trimmings and a ziptie, and ziptie it to two outer strands.
Start with one end of the inner panel. Use zipties to fasten two corners to matching spokes on both wheels. Take the ziptie straight out to the spoke, staying below the rim. The ziptie may sag toward the center of the wheel; this is okay.
The inner panel does not just rest on the inner twine surface, it gets woven over and under the zig strands, but never over a zag strand (the outer surface.)
Pick up the unattached end of the inner panel. From where the attached end is ziptied, guide the unattached end over one zig strand, then below the next three zig strands. Repeat one over, three under, until you come around to the start. (All of this done underneath all the zag strands that define the outer surface.)
Spread the tulle flat and tension it mildly both lengthwise and crosswise. (Clothespins may be helpful temporary anchors.) Anchor the tulle edges to nearby spokes, working both wheels alternately. Tension the tulle as you go and space the zipties about a handspan apart. The tulle surface, particularly the edges, must be under enough tension that it can't flop far enough to touch the outer surface. If slack areas develop, use more zipties.
The outer surface is easier. Fasten one panel end with zipties, wrap the tulle around the drum, trim the excess, tension it, fasten temporarily to smooth the panel and even the tension, and mount zipties.
My experience: This whole process is complicated and error-prone, and can take a couple of hours. If some aspect isn't perfect, the only likely consequence (other than aesthetic) is that you may diminish the performance by a small amount. Keep in mind that you build the drum only once, and then use it for years.