I wonder what the original dealers used...
Im betting a box though I have seen a picture of a set up that mounts to the points the crash bars mount on.
If you have a lift that you can strap the forks to it then make a Box.
Chris Harris Box:
PARENTAL WARNING Chris is a word smith with four letters. Keep the kiddies away.
If you look in the comments he suggests a couple lag bolts into the floor if you dont have a lift in the comments. I bet you could make a quick jig with some 2x4's and plywood to replicate the wheel clamp on a lift.
At 10 min mark he shows the box being used in his K bike clutch spline lube video
That said I used the Home Depot/Lowes yellow folding saw horse you see in a lot of pictures and it worked quite well. I never felt the bike was unstable. Since I was doing the job alone I took my mover dolly and some wood I had laying around and cribbed the transmission. I was able to easily remove the transmission. I bought some all thread to use as dowels (watch the clutch vids) to guide the transmission on and off.
After doing this once now I think I could do the next one in about 6 hours or less on Non ABS bikes. Fact is next time I'm not removing the swing arm. Final drive yes so I can lube the splines there but since I now have a permanent index mark on the drive shaft (paralever) there really is no reason to remove the rest of the swing arm if the bearings feel good.
Before everybody points out the clutch rod.....I did not take the boot off and remove the spring. My boot was in good shape. I tied the clutch arm loosely up with a zip tie so as to not stress the boot. There was enough room to back the transmission off the clutch and clutch rod and spin it sideways to clear the frame legs. ;)
Edit:Picture (not so great but you will get the idea) of my dolly set up about midway down.
http://www.motobrick.com/index.php?topic=580.925