I've been looking for a small strobe light for a while and found the Guardian Adventure Light - I also want to have a small, shoulder mounted flash light which is readily accessible, but which doesn't have to be hand held unless I choose to do so. (I'd normally use a head torch). This article shows my current "Mk I" set up.
Another suggestion for mounting something on a shoulder strap is described here.
A Guardian Adventure Light - the clip doesn't grip all that well so I've used heavy duty self-adhesive velcro to attach it to the shoulder strap.
They cost about £10 on eBay - £14 or so from dive shops. They can be set to either strobe or show a constant light.
(The orange cord is just a spare leash).
This small LED torch runs on an AA cell and came from eBay for .99p + about £6 postage from Hong Kong. (It's sold as a "CREE LED 270 Lm 5w Flashlight A20") - it's only being used to prove the concept as it is NOT waterproof, despite claiming to be.
It's really necessary to spend a bit more - perhaps the EagleTac here which has an IPX-8 rating.
A piece of webbing strap has a bit of heavy duty self-adhesive velcro stuck to it and a pair of velcro ties are then attached. Most of the DIY chains sell them.
The strap's buckle is hidden behind the sleeve on the ba's shoulder strap.
The torch is nice and secure, but still easily removed. It's position allows it to be turned on easily. The angle is just right for it to be used to light a map if you lean forward a bit, or to shine at an oncoming ferry if you're looking at it - as you would be!
This shot also shows the velcro on the strap.