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.