Like Paul, I used the provided mountings on my Pz III but I found they were too soft and flexible. Although you could use packing quite successfully, you might also consider making a mounting pad by casting some RTV silicone. I get mine from:http://www.mbfg.co.uk/silicone/1312.html
Just fit the speaker using the supplied mount, build a box around the top end using plastic or card and fill with RTV. It moulds itself around the speaker and gives you a permanent cusion. I think I put some photos in my thread but you'd have to search.
Hope this helps.