Thanks for the inspiration, I went out this afternoon and bought a "watertight" food box, its going to be a sod to paint 'cos its made of polypropylene, so I may just glue some bits of drab material to it.
I had already bought the battery holders, a bunch of cheap second hand 49mm camera filters including a UV and polariser and a voltage regulator module so to allow me to precisely fix the output from the battery pack. The plan is to also wire up an optional PIR module so I can use it either for wildlife or time lapse. The idea with the PIR is to extend the battery life dramatically, since the PIR draws about 60uA. I should be able to make the PIR switch on the regulator, and this in turn will power up the camera, which will start a default CHDK script once powered up.
My food box has two internal containers, one of which is subdivided into two, in this I will house the batteries in one section, the electronics in the other, and in the top larger container I intend to fit an adjustable shelf to allow me to use either an Ixus or a Powershot A series.
If I find the time, I will post some pics of the build, and any results of shooting.
Total build cost so far
2.99 GBP for the food box Sainsbury supermarket own brand, special offer 2.0 Litre.
6.00 GBP for 18 AAA NiMH rechargeable batteries ebay (incl. free postage from China)
4.00 GBP for the voltage regulator module (ditto)
1.29 GBP for the PIR module (ditto)
3.00 GBP approx for the battery holders (from my junk pile)
1.00 GBP approx for the power connector to fit the Canon power socket
(also from my junk pile)