Most move air roll to L1. I play claw (index finger for Circle, thumb for rest), but if you can't do that, I suggest Boost to R1. Also, powerslide to L1 isn't a bad idea (I just couldn't get used to reversing with my left middle finger).
This is a checklist of things that you should be able to do with your controls on a controller.
Gas and reverse on analog input, aka triggers.
Have a camera swivel on an analog stick
Have steering on a stick
Be able to jump and boost "at the same time" = "atst"
Be able to Reverse/Gas and powerslide atst
Be able to powerslide and boost atst
Be able to air roll and boost atst
Optional ability to be able to jump and powerslide atst (for wavedashing but there are other ways to wave dash without powerslide)
Ideally, no other function should be on the reverse/accleration key because it makes you go slightly faster/slower midair.
Air roll left and right are not necessary. They're mainly personal preference and have their pros and cons (analog input of default air roll modifier vs. binary of air roll binds), but if you want to use them for half flips, very advanced and niche techniques, or just feel like they're more intuitive, you can use L1 for air roll left and R1 for air roll right. I personally 100% don't recommend it.
Most top pros don't use the binds (some use one for half flips), but there are a few bubble players that use them. In the end of the day, I personally suggest the normal air roll modifier as I like being able to choose how fast I'm rolling (analog input), but I'm not here to tell you the binds are objectively worse (since it comes down to preference).
My two/three proposed set ups
#1 (Most recommended)
Jump: X (A)
Boost: R1 (RB) * Use right index finger
Powerslide: Square (X) or L1 (LB)
Air roll: L1 (LB) * Use left middle if using L1 for powerslide
Gas: R2 (RT) * Use right middle finger
Reverse: L2 (LT)
Jump: X (A) * Fat thumb X and Circle/Square
Boost: Circle (B) or Square (X)
Powerslide: L1 (LB)
Air roll: L1 (LB)
Gas: R2 (RT)
Reverse: L2 (LT)
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