Just finished the mobile base for my drill press, but it could be adapted for any tool. I forget where I got the idea from, either here or Woodnet.
The main goal was to raise the working height minimally. It's probably slightly over-built, but better than under-built I guess. The tool base is bolted to a sheet of 3/4" plywood that is oversized 3-1/2" (2x4's width) in each direction from the tool's base. I then built up the height using 2x4's until the casters could hang. All the 2x4's were then bolted and glued to the plywood sheet. So the load is being transfered through the bolts, to the top 2x4, and then to the casters. So that is why I have so many bolts.
I really like this base better than the Woodcraft/plywood bases I have on my other tools. Especially since I was able to extend the tool's footprint in what I considered appropriate directions for stability purposes.
Of course, before I actually placed the tool on it, I did the appropriate amount of scientific load testing. Both my wife and I stood in the middle and bounced. Together we are easily over 350lbs, so I knew the 275lb drill press would be held fine.