The base Geoff Andrews made for his calliper modification in the latest MEW magazine reminded me of the shape of the (very) cheap and cheerful version I use for depth measurement.
I bought a digital tyre depth gauge (about £6). It uses exactly the same electronics as the cheaper callipers mated to a stainless steel pin. After milling the reference face flat ( its only plastic), it makes an excellent little 0-25mm depth gauge reading to 0.01mm and meets 99% of my depth measuring needs.