I've seen these for sale but thought it would be a nice easy job to make ureself its also a nice milling exercise for a newbie that will give a usful addition to your kit.
This is it

Its made from a bit of 1/2" mild steel plate.
First job is to check you're mill for tram and that you vice is set square as any errors will be passed onto the gauge.
Next square up your chunk of plate and mark it out.

Then drill your holes

Then set up nice a flat on parallels with the end sticking out from the vice.
First mill the slot in the midle to take your vernier make the slot just a little bit deeper than the thickness of your vernier, and clear away for any obstruction on the vernier that will stop it sitting flat, mine had a screw standing proud.

Then without taking the base out of the vice mill across the base this way the base will be dead square with the slot.

Next cut away all the unwanted meat and give it a bit of a polish, I skimped the polish a bit as it won't work any better no mater how shiney it is, I'm a bit lazey that way.
Then saw through to the midle hole, then drill and tap a M3 cross hole and open it out 3mm for half the distance to make a pinch clamp.

And this is it in use

It measured the 2 inch side withing 0.0015 depending where I positioned it on the table, it was picking up bumps and hollows on the table, the table is one I picked up from the scrappy and its seen a hard life.
Cheers

Stew