Secondary with Brett question

Last night I racked a black saison into secondary, pitched Brett Lambicus, added oak, and some of my favorite dregs. I thought I had three gallons of beer, but in actuality only had two (don't ask why!)

I racked the beer into the only fermenter I had: a three gallon better bottle carboy. But I'm a tad concerned about all the head space and getting too much oxygen into the beer.

I do have a two gallon glass jar. The beer would fit in there, and I could then tighten the lid. However, I'm nervous about re-racking the beer, and I don't particularly like the idea of an air-tight lid on a glass jar; even though all my brett-in-secondary beers have had very little visible fermentation activity, I still think I could wind up with an explosion.

What do people think? Should I leave it in the 3 gallon and not worry about it? Should I transfer to the 2 gallon jar? Or should I order a proper 2-gallon carboy and transfer whenever it finally arrives?

What dregs. I'm going to assume some sours and now you have lots of bugs, and this will be there for the long haul.

Either rack to the 2 gallon and put an airlock on it (don't seal it in), or add some recently brewed beer to top-up the 3 gallon jug (and put an airlock on it.

Perhaps you could drill a hole in the jar lid and place an airlock?

I'll sometimes add a little table sugar when I have a bit too much head space. Not enough to change the flavor but enough to produce some Co2.