I use a Simpson Voyager at the moment and it is fine. It is quite a large helmet even in the correct size but I have no problems using it with the roof up or down. This is in the UK and I was advised that in the US I would not be able to use it as it is above the roll bar and I would need to use an extension to the roll bar like the BK one.
Something else to check into, or just keep the roof up, or your hardtop on. I love being on the track with the roof down
I really like the look of your car in your avatar picture, my hardtop is being painted now.
Cheers