Here's what came out:
Its not very lightweight due to the battery, but easy to carry, loud, I hope quite robust, and versatile. It has a 5A fuse.
And it looks a little bit like Orac, if you ignore the red beauty case I got from from a recycle shop, but not many people will know Orac.
I ended up using two of these little timer relay devices, one for the 20 second countdown and 5 seconds of light, and another, triggered by the first, to keep the beep/car horn limited to 0.5 seconds.
Its fun to use in the boat, although there were some concerns about hearing damage from the ones operating it. With just the beeper activated, not even the car horn. I didn't tell them they could disable the sound, yet.