If tailored gun cases stretch the budget try one of these RAF pilot bags, made by Rogue in South Africa and sold in the UK by Kendrick Imports, it fits neatly behind the seats on a trad. I've bought one and intend to try it out across the Channel soon.
http://www.kendrickimports.com/catalog/product/gallery/image/600/id/539/

Last edited by Easter; 28/01/12 05:48 PM.