I've found that running undersized tubing is enough to hold in most situations under 18psi but why take the chance? Get a box of clamps from Mcmaster. Any clamp designed for that diameter range are fine.
I agree with Bobby. Why take the chance? My co2 and beverage lines were all so hard to get on (and even harder to get off) that I'm relatively certain they'd not leak under any normal use. But I still use stainless worm clamps on every connection. I just got mine for a few cents each at Lowes. Cheap insurance.
Tap 1:Traditional Bock Tap 2:Robust Porter Tap 3:California Common Tap 4:Old Ale Tap 5:IPA