Some clever people on the Isle of Wight have come up with a tracking method called a vole spoon, which I thought was incredibly neat. They recycle old plywood blades off wind turbines, smear them with a layer of mud and push the pointed end into the wet river bank at water level. The 'spoons' are baited with a bit of apple and left overnight, and if water voles are about, they should leave tracks. The whole thing's a lot more portable and discreet than a mink raft, it has to be said.
More information in the Island Trust's newsletter: http://www.island2000.org.uk/newsletters/2008mar.pdf
and here's a spoon in situ: http://www.flickr.com/photos/island2000conservation/2386392369/
A similar size and thickness of palette could be fashioned without too much difficulty, if you don't have any spare turbine blades knocking about!
Update: I've been talking about the vole spoon on the excellent Wild About Britain forum, and several points came out of the discussion that I'd like to use to clarify the information here.
Firstly, it would be crucial to site the spoon somewhere it wasn't exposed, because obviously the last thing you want is to encourage predators to pick off your water voles as they feed. Nestled down among tall reeds, for instance, might be a good spot.
Secondly, the spoon, as I understand it, is meant to be used infrequently - at most, once a season per stretch? - and for a day/night only. If you left it in place permanently, it might again encourage predators to pick off your voles.
Lastly, it's not meant to be a replacement for survey work. Surveys which count and record field signs like latrines and burrows and feeding stations are crucial in monitoring water vole populations, and in getting to know particular areas and habitats. However, there may be occasions when you need proof of water vole presence quickly - for a building developer or a council, say, or after a bad flood - or when you can't examine the full length of a bank because it's mostly inaccessible.