What about adding lettuce and/or tomato?
um... I think the problem with lettuce is that the baloney is really smooth, so + the lettuce, would make the sandwich unstable in the grip..
But, i guess it'd work well with Chopped lettuce, or maybe if you got the lettuce very dry, and put some cuts into it to increase friction..
Tomatoes haz the same instability issue, but it's ok as long as you got enough mayo for glue.