What is Reddit's opinion of Anderson Metals - 57002-0604 Brass Hose Fitting, Connector, 3/8" Barb x 1/4" Female Pipe?

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Here's a bigger swivel nut, they do exist.

There are other solutions, including:

• A bulkhead post that you can install instead of that blow-off.
• A 3/8" barb that will fit directly into the existing silicone bung. Then add a post adapter and a gas post, and you have something you can swap for the airlock (without removing the bung) after fermentation is complete.

For the Speidel (and other vessels not designed to hold pressure), I think most people probably just swap it quickly. When I had a Speidel I didn't use the oversized bung/airlock... I replaced them with standard ones and then picked up one of these (which fits in a standard drilled bung), and attached a keg post to it. It still required swapping, but it's a very small hole.

For pressure-rated vessels people will often swap in spunding valves near the end of fermentation while there's still some CO2 being produced (something like this). It might work to dial it back to like 1 PSI or something... personally though I wouldn't trust leaving a fermenter under pressure for an extended period of time while unattended if it wasn't designed to hold pressure.

Not exactly what you're asking for, but I've made one before with this and this and a gas post.

I have one of these with an extra keg gas post screwed into it. The 3/8" barb fits in the bung, so you can pull out the airlock and just replace it with that, and then you can use a normal gas quick disconnect.

This might work too, never seen it before.

I picked up this and this and an extra gas post to do this.

That barb fits the same size hole as a standard airlock. When cold crashing or pressure pushing I just pull out the airlock, replace it with those pieces, and connect the gas QD.

It works well in a bung, but didn't work great in a grommeted lid since there isn't much to support the barb... I was able to get it to mostly work by adding some keg lube and holding it straight while pressure pushing.