Crackers probably best avoided to be honest as they are still relatively high carb.
You could use lettuce leaves or even use the contents of the sandwich.. like 2 slices of cheese with ham in between or 2 slices of ham with cheese in the middle. Bread is usually only the carrying medium for something more tasty so just eat the filling without the stodge. You could even butter your cheese if you want to add a bit of extra fat.. or is that a bit weird even for me.. nope I've done it.