Go to your local Lowes, Home Depot, Ace and buy 1/2" foam backer rod.
It is made to fill gaps around door jambs and window boxes prior to trimming with wood molding.
It cuts to length easily with a razor blade or utility knife.

I use it in my 450/400 with no issues cut to be lightly compressed between the powder and the base of the bullet.

It is pretty much exactly what Kynoch uses but needs to be cut to length.

This thread shows the stuff and how I am using it in 450/400

http://forums.nitroexpress.com/showflat.php?Cat=0&Number=257209&an=0&page=0#Post257209