I have designed this in response to those thoughts in hope of advancing the process of making carboys into barrels and making better lambics across home brewed America.
By presoaking the wood in water until it submerges my hope is that it won't expand very much once it is in contact with the beer. The Rubber liner is also designed to give the wood a little extra room to expand before the carboy is cracked. If pressure build up is a problem and beer is being forced up through the wood then simply adding a channel for air to escape and an airlock should solve that problem. Oxygen and CO2 should still be able to travel in and out through the wood while excess pressures will be dissipated by the air lock.
(read more after the break)
Making this is as simple as putting a piece of untreated oak onto a drill press and a lathe. This isn't a wood working blog so I'll spare you the boring details of how to make this; if you know someone with the tools chances are that they'll know how to make this, ask for a favor.
Version 1.0 variants
For a proof of concept I had three of these made for me by a friend. I don't presently have any woodworking tools and I'm not sure they'd be allowed in my studio apartment. He left the bottoms a bit longer than my original paper template which I've decided to deal with in a few different ways to test out what the best techniques are here.
The first one is a long cylinder which I'll just leave as a long cylinder and deal with, I had to drill a hole near the top to allow air in the head-space to escape even though beer will be filling the bottom hole.
The second one started as a long cylinder but I cut it length wise to expose the whole channel and still allow some oak to dip into the beer.
The third one I just truncated as in my original design. This one won't touch the beer and may dry out, it may or may not have the same properties as an oak barrel because oak is constantly in contact with the beer where as this will not be. I'll also have to monitor this one for drying out and make sure it is firmly in the carboy as it will start out wet.
I am trying these out in all three sour beers I've brewed(one each), if the technique is successful it could be a good and sustainable alternative to other oaking products with the potential of giving more styles of beer the authentic aged oak taste. At one point all beer was aged in oak casks, bringing that flavor back to my home brews where appropriate might be that unique flavor which sets my beers apart. If one of the above form factors seems to work better than another I might redesign this and make several more to the same form factor.