Pottzy, I'm not toolmaker either though I often act a tool... the cuts are simple and the parts are easy to acquire.

If you are looking at a bought alternative, it'd be hard to bypass that Paulini Pocket rule from Woodpeckers (the small one)... which I use a lot for small parts measuring because of the stop block and have found that I tend to use that more often than my jig... however, as the stop block is quite narrow, I still fall back on the jig when I need more accurate markups on non flat/irregular surfaces.
Nevertheless I have a real sense of pride in making this and I'm sure many would take pleasure in making one similar, just for the sake of the exercise... and it's one of those tools that has more functionality than initially planned.

Problem is I can't imagine you, or any imperial Philistine, making one, as it is based on an Aussie 10¢ piece and the length is governed by a metric ruler... at least your currency isn't "£.s.p".
 

If your first cut is too short... Take the second cut from the longer end... LBD