You're not going to find many of these, sadly. There aren't many clear plastics that play nice with high pressure. You can MAKE one of transparent ABS, but that has a nasty habit of exploding under too much pressure.
Which competition is this? Would a glass or plastic porthole into the tank be acceptable? A sight glass is a very easy way to add visibility to a process. They're designed to add a visible port to watch oil levels in systems where that's required.
https://www.amazon.com/Compressor-Fi.../dp/B00880DXQC
https://www.amazon.com/Thread-Diamet...YG6E 2RZQG0H7
https://www.amazon.com/W-Anderson%C2...7KC8 D5N41ADP