I made an extra grate for the bottom section and a small one for the top section for my WSM 22.5. All supplies ordered off of Amazon, so you may be able to find better prices at your local hardware super-store.
They look basically like what Len just posted, but just sit on top of the grate, and lift off. Nothing to attach or detach, just load grate, add grate, load grate, add grate, load, add, load.. Sorry, lost track there. Same concept as other people using bricks, beer cans, chunks of wood, etc... Only more stable and re-usable.
Total cost of project = $61.44
U-bolts $29.97 for ten delivered.
http://www.amazon.com/exec/obidos/ASIN/B005IZ3TS2/tvwb-20
25 nuts for $5.38
http://www.amazon.com/exec/obidos/ASIN/B004XMWLPS/tvwb-20
25 washers for $7.61
http://www.amazon.com/exec/obidos/ASIN/B009OL4W8C/tvwb-20
17 inch charcoal grate for $10.99
http://www.amazon.com/exec/obidos/ASIN/B000WEKNMY/tvwb-20
13.5 inch cooking grate for $7.49
http://www.amazon.com/exec/obidos/ASIN/B000WEIKL0/tvwb-20
These particular U-Bolts are "square" so the business end of them are flat and make a nice sturdy leg to sit on top of the other grates. Just screw the nuts/washers up through the bottom of the grate. Doing three "legs" per grate, spaced evenly, assures that the angle of the "legs" never line up with the slots of the grate it's sitting on, so always has a secure surface.
I used 3 U-Bolts for "legs" on both grates.. And two U-Bolts for "handles" for the lower grate addition. If you wanted to skip the top grate/rack, you would just need the 17 inch charcoal grate, 5 U-Bolts, 10 nuts, and 10 washers. Didn't price these out at these quantities, but I'm guessing you could do the bottom grate only for around $30-35? Or, if you wanted to do just the top grate, would require the grate, 3 U-Bolts, 6 nuts, and 6 washers. You could probably use a little bigger grate than he 13.5, but I couldn't really find anything in the 14-15 inch range.
I have pictures, will try to load them here if anyone wants to see. I doubt I'll ever use all for grates(two stock and two home-made)... But it was just one of those things that I kept out-thinking myself on.