I use cassettes made from cable trunking, just right for N gauge, not easy to carry more than one at a time though, need both hands, so hunted around for a suitable box, I found this

http://reallyusefulproducts.co.uk/uk/html/onlineshop/rub/b26_0litreTrayPack.php

next thing was to make a wooden tray so that I could get  a second layer, the box came with 2xA4 size plastic trays which sit in the top and will be handy for "loose" stock not in cassettes.


Box with cassettes on the bottom, a wooden tray with more cassettes, and nearest camera the plastic trays

I have now added foam under the wooden tray and the top trays which should hold stock in place on the cassettes so this is how a couple of Dapol Maunsells look (the box is not completely clear hence the foggy view)



I still have some foam left in case more is needed! Also have to line the top trays, there's an Ebay seller that does A4 inserts for N gauge stock.

The cassettes are about 30" long, room for 4 coaches a van and loco, or 5 coaches, or similar length freight, there are also some shorter ones for push-pull/local traffic or shorter freights, blocks of foam can be added either end of a train to prevent any fore and aft movement