One solution is to use a small 12-volt motorcycle battery and power the indicators - through a fuse - from that. On my cars, the indicators are all earthed back to the battery and, with 21-watt standard bulbs, are very bright. The units I've used on PL3002 are the standard type sold for many years - a sort of mini "Rubber Light" that comes with a simple, right-angle bracket. I've fitted these to PL3002 on auxiliary brackets secured to the front wing stays and, at the back, on the number-plate support arm. Willie McKenzie has them on this page:
http://austinrepro.com/headlamps/ Lots of people have used these. I saw them fitted on David Wortley's cars and tried to copy his especially neat mounting arrangement.
Rather than the massive switch provided with the indicators, I've used a small, less obvious one and connected to it is a loud warning buzzer
https://www.vehiclewiringproducts.co.uk/p-637-cab-buzzer?search=buzzer It's only available as a 12-volt but it does work on 6 - though not as loudly of course.
Connected to the same circuit as the buzzer is a tiny "grain of wheat" but very bright red LED light that really easy to fit anywhere - mine is just trapped against a tiny piece of wood slotted to "clip over" the inside edge of the driver's glove pocket.
Something like this does the job 6-volt are also available.
The switch is available from:
https://www.vehiclewiringproducts.co.uk/...-on-switch.
For how long does the 12-volt battery run a 21-watt bulb continuously? About 5 hours - that's lots and lots of turnings into junctions.