Using the Isle of Wight 'vole spoon' as an inspiration (http://staggsbrook.blogspot.com/2009/01/vole-spoons.html ) Lorcan from the Whitchurch Water Vole group's come up with the Whitchurch Plate, an overnight tracking device designed to gather evidence of water vole presence quickly. He's used a metal collar with swivelling legs at one side that can be sunk into the bank either vertically or horizontally. The plate - an ordinary dinner plate - is spread with soft clay in the same way as a mink raft, then balanced on top of the metal frame. Then you add a few bits of chopped apple and leave for 12-24 hours (maybe 48 maximum). The whole contraption's attached to a bit of string so you can pull it out easily, doing minimum damage to the bank side.
To improve the model further, we need to attach the plate securely to the collar - use a plastic plate with holes drilled in the sides and fastened with wire, maybe. We also could put a thin layer of soaked oasis under the clay to keep it moist, as the mix I spread on the test plate dried out slightly and therefore we only got muddy footprints from the voles rather than indented ones.
Still, essentially it works, and is a LOT less cumbersome than a mink raft!