What is Reddit's opinion of Paksh Capitol Glass Decanter with Airtight Geometric Stopper - Whiskey Decanter for Wine, Bourbon, Brandy, Liquor, Juice, Water, Mouthwash. Italian Glass | 23.75 oz?
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You can find some pretty cheap glass decanters with rubber stoppers on amazon, such as this one, which I recently bought.

Two important things to keep in mind when shopping for decanters for an infinity bottle:

Make sure it's made of glass or lead-free crystal. Some crystal is made from lead, which will leach into the whiskey after some time, and is not really safe. Glass decanters have no lead, the product specs and/or Q&A should tell you the material of any decanter you're looking at.
Make sure it has a rubber stopper. Some will have only glass tops, and glass on glass does not form an airtight enough seal for long-term whiskey storage.

And of course, always read the reviews - both of the product and the seller!

Looks like I'm buying Popov and pouring it into a decanter.

I found this https://www.amazon.com/Paksh-Novelty-Decanter-Airtight-Geometric/dp/B01M0JNJGO/ref=sr_1_8?dchild=1&keywords=whiskey+decanter&qid=1600261784&sr=8-8 on Amazon. Looks classy, seems to have enough flat glass for engraving, and most importantly has a plastic seal around the glass stopper. Depending on the number of guests you may need more than one!

$50 - $100

< $50

< $50