Having looked at the design of them, they don't look too difficult to make at home. A thin sheet of plastic from a local sign maker is all you'd need.
 Found this thread and a link to a drawing
An alternative would be a Paterson Orbital tray - I've started using one for my 5x4, and find it much easier than a Combi-Plan tank or regular tray. The Patersons turn up fairly regularly on ebay, all be it at silly prices, and occasionally with the s/h camera dealers - I got one for free, and another with a motorised base for a fiver.
Last edited by paul_c5x4; 06-15-2009 at 01:34 PM. Click to view previous post history.
Reason: found a link to a previous thread.