Although this is a fairly good project idea, why re-invent the wheel? Most auto parts store -- and any truck parts outlet -- will have electronic turn signal flashers available. These are considered by most truck maintenance experts to be required exuipment in place of the older electro-mechanical relays, as they are non-load dependent. Consider the differences between the turn signal or hazard warning circuits of a bobtail tractor, a tractor/semi-trailer combination, a set of doubles, and a set of trips....
In the USA, the most popular brand is
Grote, and they offer a 2-prong version (P/N 44530) and a three-prong version (P/N 44540), catalog info **broken link removed**. The electronic flashers are plug-in replacements for the electro-mehanicals, but if your flasher(s) are encased in a fuse panel etc., you may want to remote-mount them and use an extender harness to plug into the panel, as these flashers are somewhat larger.
These flashers have been available as optional factory equipment on medium and heavy trucks for years, and when Chevy started sticking a string of signal lamps across the back of their Camaros, electronic flashers were installed as factory equipment there as well.
Your profile doesn't indicate your location, but I'm sure that youo can find these babies or their equivalent somewhere...