What about taking a small piece of 3/8" tubing (just enough to cover the 3/8" barb) and then put your 1/2" tubing over it?
That's what I did with my sparge/sprayer tip. 3/8" vinyl tubing and my 1/2" Silicone tubing slides right over it. Secure it with a clamp and your done.
Probably something you have laying around, cut 1/2" off you racking tubing... a free fix!
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