Any upholstery shop will sort you out with foam of any density to suit your preferences.
To be honest I thought that was altogether far too much hassle so I went to a sports shop and bought a couple of neoprene exercise mats made of a low-ish density 10mm neoprene in an amusingly foppish shade of lilac. I cut off a couple of bits for the bottom of two 84L Really Useful Boxes and cut the rest in half lengthways and zig-zagged it to make pockets for pads and cymbals to sit comfortably and safely in. I'll get pics as soon as I can.
In the meantime there's a pic of my old kit packed in something exactly similar (but with a black mat instead of lilac) here
It's not terribly sophisticated and you wouldn't want to take it on a plane or anything, but if you're just chucking them in the back of the car it's just the ticket.
BTW - I used a few sets of the interlocking exercise pads (****s Sporting Goods) to make an inexpensive floor pad for my TD-12S. The rubber encased tiles did a great job of protecting the floor as well as keeping my kick pad from jumping about.
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