Harris but with a different middle name as I agree that it’s not ideal with Harris. Could Harris Finnegan work?
I like Arlo but I think Harris has more staying power and I like the presence of Harris (and the possible nn Harry).
You could even best of both worlds -
Eg
Harris Logan nn Arlo
Harrison Milo nn Arlo
Charles Olivier nn Arlo
But if you go for Arlo Finnegan, it’s a handsome choice. You can’t lose (so long as, imo, Harris has a new middle 
).