I use a stainless steel blowoff tube in my carboys in order to fit in the mini-fridge I have. They are angled specifically for glass carboys. There are SS blowoff tubes for buckets too. I bought a pack of three to use on my bucket and fustis. What I found was that while the angle was better for a flat top, they don't stretch far enough to cover the distance from the interior of the lid top to as an example a grommet centered in the lid. However, I have added a thermowell to my bucket (and will do so on the fusti lids) so I just put the thermowells in the original grommet holes and then added a new hole closer to the side for a grommet for the blowoff tube/airlock. It's not particularly necessary to have the blowoff tubes on my bucket or fustis when using the mini-fridges as they are shorter than the carboys but the holes were too close to the middle to use the SS blowoff tubes.
Note the main difference between the carboy blowoff tubes and the bucket ones I have are that the bucket ones have less horizontal travel and are shorter vertically. Perhaps there are different manufacturers so what you want with a flat lid is something that has a flatter angle and some horizontal travel. The smaller, lower one is the "bucket" blowoff tube.
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