Only a few days after initial completion I was able to make some more progress. The biggest addition was my use of some extra brackets to add a spot for a keyboard on a swinging arm. I also did away with the cardboard by putting some felt pads on the base, which makes the entire rig easier to slide around.
It's nice having the keyboard on a swinging arm so it doesn't get in the way when when I'm settling in. If I keep this mounting solution I will likely use some Sugru (self curing rubber) on the bottom lip of the brackets to help with positioning and grip to make the keyboard truly stable in this position.