Rod and Mallet
Since you don't have a c-frame, you don't have the steel rod that goes in one. No problem. Run out and get a steel rod about 10 - 12 inches long. Diameter isn't crucial, about 3/8" will suffice. In one end, drill a hole that will accept the insert end of a dimple die. Drill that hole straight, or have a machine shop do it for you. Once you have that, simply insert the male die. Screw that Cleveland tool onto your work surface and place the rib hole you wish to dimple over it. Use the steel rod with the male dimple die in it to hold the rib in place. It should be light enough, if not, get someone else to hold the rib in place.
Ready? Hit the steel rod with the mallet once or twice to make the dimple. Use moderate force. You will visually be able to see when the dies have both bottomed out on the aluminum and the dimple is complete.
You will only need this device on a couple of holes.
Alternativelely, buy the C-frame and use the rod that comes with it. Or, there is a Vice-grip type dimpler that works well for these holes.
Best of luck!