I'd just get one of these
It will run from about one one led (0.02A) to 20 amps of mixed or incandescent bulbs
nae bother at all.
I made my own with a 555 and mosfet but had I seen the 99p ones from HK
prior to this, I wouldnt have bothered, you couldnt make them yourself for that price
Adding a load resistor to run leds on a normal flasher unit will work
of course but it kind of negates the power saving aspect of using leds
and your still using 42 watts, albeit intermittently.