Just use some fairly thick foam. Rubberized weatherstripping works well - it's about 1/4" thick, firm, and yet molds itself to whatever cells you use. And since it's ribberized, it forms another electrical insulator.
Well, I've been using this stuff (first item on page) and it works very well. But, I've only found this exact stuff at Menards. Lowe's has something similar, but it's not quite as firm.
What's nice is that it is cheap and plentiful so replacing worn out foam is easy to do.