The bottom of a SlikStik isn't exactly cardboard -- it's more like 1/4" particleboard. Not sure of the name of the stuff, but it's like the material they use in pegboard.
When I attached my SlikStik to an Atari System 1 cab, I ended up mounting the bottom board from the Slikstik to the cab (using two small L-braces, and two 1" bolts with two 1" washers) then setting the SlikStik into place and adding two L braces on the back. Solid as a rock.
If you're concerned about the bottom board on the SlikStik, you can always mount into the bottom edges of the side panels on the SlikStik, which are 3/4 MDF.
Kevin