It might be easiest to get round ones secured with a bolt that will just fit the widest point. If it is any help I have a set of 4 with an internal diameter of 75mm that I took off my car you could have for nothing, although they need a polish!
Hi Spammy. I believe you have an M54 2.5i engined E85. The M54 2.2i and 2.5i have the same size tail pipe trims and the part number is 82120151089. The 3.0i cars had slightly larger pipes. They are oval in shape, slip on easily over the existing pipes and secure with a small grub screw underneath. I sprayed mine gloss black with engine enamel
https://www.lllparts.co.uk/product/bmw-exhaust-tip-genuine-bmw-82120151089/id/466581
I have the oem chrome ones on my ovals, 3.0i pre facelift. Didn’t slip on easily, though — had negotiate them in place with a plank of wood and a hammer, 18 years ago. They have aged well, light polish every now and then and they’re good as new.