I have recently fitted a warning triangle and umbrella clips.

I rather liked the idea of a Kojak type beacon to slap on the roof, with a big magnet, as an extra warning device but the bulk is a deterrent.
Realising that things have advanced since those days I searched and found a more practical version.

A magnetic LED warning light / beacon. The unit has a battery and 16 orange LEDs with nine different flash programmes including S O S.
It has a powerful magnetic base ( also floats). Approx £15

A version, rechargeable by the 12V cigar lighter or 240 Volt UK is also available ( probably available for other countries). Approx £25

I mounted it in the boot ( trunk) lid where it is easily reached and always available.

Fixing it was quite easy. I drilled a hole through the bootliner and into the central rib of the boot lid.
I eased the lower and two side poppers that hold the lining on.
I pushed a small plastic mushroom headed plastic plug into the hole by reaching under the lining and then put all the poppers back in.
I found a reasonably thick steel washer approx 3/4" (20mm) diameter with a small hole in it. I drilled a countersink in the washer to take a self tapping countersink screw. (This is to created a flat surface.)
Then just used the screw to attach the washer to the pre-drilled hole.

The beacon can then be positioned by just using its magnetic base against the washer in the boot lid.

Can be used in its storage position, attached by magnet anywhere, on the ground or even hung up.