There are a few builders who have built nice and sturdy dimpling tables, to facilitate dimpling skins with the c-frame bench riveting/dimpling tool.
Such tables are really a great help when dimpling large skins by yourself, otherwise you would have to try and support the entire skin on a surface the size of a dimpling die.
For example, here is one that is built into the work bench (Sam Buchanan’s):
I would like to propose a softer approach to the same problem: A dimpling mattress. This is a good time of year to get camping mattresses (I am talking about the 3” foam ones, about $10 or $15 at your local Fred Meyers/Wal-Mart/k-mart).
The foam is quite soft, and I don’t know how comfortable it is to actually sleep on, but it can be very handy in supporting large skins for dimpling.
I got one of these, and cut a rectangle out of the middle, to fit the foot print of the c-frame.
Depending on the version of the c-frame, you might need to attach a shim to the bottom of the tool, to adjust the top of the dimple die so that it is just below the top surface of the mattress. This way, as you slide the skin over the mattress, it is protected from being scratched by the (male) dimple die, and a little pressure makes it contact the surface of the dimple die. I think I used a piece of ¼” plywood from the cull bin at Home Depot.
You can, of course, place the c-frame anywhere around the foam, to any position that works best for the dimple at hand.
Later in the project, I ended up cutting the mattress to smaller pieces, and using them for various other purposes, such as protecting assemblies from aluminum shaving scratches while they rest on the bench, or to support my weight while working inside the aft fuselage.