Pumpernickel is good for most type 2 diabetics, having a low GI between 30 and 40. It's more compressed seeds than leavened bread and is consequently very filling. It travels well too!
I have open sandwiches at home but if travelling, I fill the sandwiches with sliced cheeses like Emmentaler or sliced meats or salamis. Just add some crunchy lettuce and some sliced tomato and or onion. You can have a nice meal if you take some broth in a thermos and some nice pickled gerkhins and/or green chillis. An apple or pear won't hurt you either.
This is a very nice sandwhich, cream cheese, smoked salmon and some avocado, but a bit of a luxury and almost as expensive as Greggs:
Still, a good ham sandwich is hard to beat:
You need the correct bread though. Nothing light and springy. That's a sure sign that it has white flour and/or other additives. Pumpernickel in German is called Full Kernel Bread and it looks like this: