Yep I was having a thick moment, I blame low blood glucose, yes m5 its the only size that'll fit through the spaces in the battery rack, its been a long time since I fitted my battery rack lol - I did want to avoid buying such a vast array of rivnuts as this will be a one time application and I haven't looked yet but I can guess it'll be cheaper buying a small pack of rivnuts, some to practice with and a few spare.same as your existing bottle cage bolts
Thanks for the response!