From Brewers Hardware:

Introducing our new line of clear Tri Clover Compatible caps! One disadvantage to using a stainless steel fermentor is the inability to see what is going on during fermentation. These caps solve that problem!

Made completely of clear polycarbonate, these caps are extremely durable and allow you to view inside your vessel without removing the lid.

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Check out the full lineup at Brewers Hardware – Tri-Clamp/Tri-Clover Caps

Also: Brewers Hardware has a gigantic selection of Tri-Clover Compatible Fittings including some really unique offerings