The smallest would probably be another AI computer unit, something like the oceanic/aeris computer mask or another VT3 computer left in a pocket for backup. However, I assume you are looking for something cheap yet reliable? Basic analog glass and brass SPG with 2" face can be had for about $60, give or take. Many people like to tie a stainless steel boltsnap to them and clip them off to a d-ring located at the left hip area. There it's out of the way unless needed. Analog depth gauge cost about the same and you could leave it in your pocket, or somehow tie one to your SPG. Of course you'd need a backup timing device too so a dive watch with depth gauge might be another way to go. I have seen SPG's with 1.5" faces, but they tend to use plastic instead of glass, which some prefer for deco bottles because they are lighter, but for a backup SPG to your primary I'd go with brass and glass.
