Rob
We use these for another purpose
[link=http://www.ebay.co.uk/itm/Timer-relay-board-adjustable-10-100-seconds-in-10-second-steps-/110833008899?pt=UK_BOI_Industrial_Automation_Contr ol_ET&hash=item19ce298103]Timer Relay[/link]

The boards are 12v, but this is mainly due to the 12v relay it could be replaced with a 6v relay, there are loads of similar boards commercially available, they about the same size as a reciever.

If this dos'nt suit, if you let me know more specifics, time etc then I could make you one up easily enough, switching a mosfet instead of a relay would probably be better(smaller) for model use as well.