Yes I couldn't find a cap for the barb on the fill either. I simply filled the inside with epoxy and it seems to work good. I have 3/8" tubing and used a 1/2" T to avoid causing adding too much resistance. I use a lot of 1/2" connections and then force the tubing on over them. It adds a little security as those things aren't coming off without a fight. For your purposes, trying to fit a 1/2" hose on a 5/8" barb might be easier then a 3/4".
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