The first time I tore apart a front shock I was a young auto mechanic apprentice. My tools were minimal but 2 of the first tools I bought was an impact gun and an air ratchet. I can appreciate the fact that a lot of motorcycle guys that do their own work don't have an air compressor so an air impact driver is out of the question but their are a lot of electric impact drivers that work quite well.
So, think about removing a wheel from a car- If you use a tire wrench, you have to hold the wheel from turning when using the wrench. Usually you break the nuts loose with the wheel on the ground, then lift the vehicle and remove the wheel. if you use an impact , you can have the wheel off the ground and just break the nuts loose and remove.
So why did I digress back to this example?? Same principal can work here when you don't have the "special tool (cylinder holder)" What I do when disassembling a shock.
With the shock on the bench, the first thing I do is put an impact driver on the Allen head screw in the bottom of the shock and break it free and remove it. Why do this first?? The compression spring in the shock sits on top of the inner cylinder which is the part the Allen head screw is threaded into. The friction of the compression spring as installed, holds the inner cylinder with a little force to offset the turning of the Allen screw with an impact gun. Once the Allen head screw is out, continue disassembly of the shock and do what work you need to.
When reassembling, reinstall the inner cylinder and install the Allen head screw but don't tighten it down until the very last thing you do. The Allen head screw will hold the inner cylinder in place so you can reassemble the rest of the shock and once all the pieces are in and together, tighten down the Allen head screw