Tried all that, just being mean I thought I would augment my Bowens by using the two 600 EX RTs - after all in studio mode they just sit in the bag!
A friend tried numerous optical triggers, with variable levels of success - to me as I have Pro Bowens kit and the bowens pulsar TX radio triggers if I press the shutter it must flash every single time. I just can't be bothered with hit and miss crap.
I did go the STE-3 route and that makes the 600s flash everytime, and that is my studio radio trigger backup if god forbid the Pulsar TX system died on me. {NB I had to have the 600s for outside work.
Actually I came across some real low tech Yongnuo 560 IIs £40 delivered on Amazon, GN 58 ? [my a*s*] more like a little more powerful than the 430IIs. BUT for targeted additional lights, where say you can't stick a thumping great Bowens Pro moonlight and a wide angle reflector, and power cables etc. they WORK EVERYTIME ON 'S1'.
Now got two!
For us brits it's about the same price or a pound cheaper; as a Flash Zebra Cell and adapter and postage! [that my friend says is the best]
AND THE ICING ON THE CAKE IS THEY ALSO WORK ON S2 with the 600s being triggered by the STE-3 even in ETTL2 mode!
Summary - for the same price as a flash zebra photo cell trigger and the required adapter, I get a fully functioning speedlight - that is fired from the bowens studio flash, no wires or attachments; as its photocell is built in. And they can be fired from the 600s in radio mode from the STE-3 trigger.
Canon EOS 5DS R, Canon EF 70-200 F2.8 L Mk II IS USM, Canon EF 70-300 F4-5.6 L IS USM, EF 40mm F2.8 STM , RC6 Remote. Canon STE-3 Radio Flash Controller, Canon 600 EX RT x4 , YN 560 MkII x2 ; Bowens GM500PRO x4 , Bowens Remote Control. Bowens Pulsar TX, RX Radio Transmitter and Reciever Cards. Bowens Constant 530 Streamlights 600w x 4 Sold EOS 5D Mk III, 7D, EF 50mm F1.8, 430 EX Mk II, Bowens GM500Rs x4