Scope trigger circuit

Anyone know if there's a ready-made box that:

1) Accepts a standard 1M 10X probe on a BNC 2) Gets some external power or better yet, runs on a 9V battery 3) Has a nice pot that can set the pulse length

I want to add the useful "pulse" trigger function to my Tektronix 547 scope.

You know, only triggers on pulses longer/shorter than "x".

Look, I could probably bodge something together, but does such a box exist? I'm lazy.