I have a 3-carboy solera I use for Flanders Red.
The first carboy is the newest beer with a standard rubber stopper and 3-piece airlock. The beer stays in the first carboy for about a year, with 1 oz of medium-toast French oak cubes for the last month or so. I just drop the cubes in some boiling water for a few minutes to sanitize them before adding them to the carboy,
The second carboy is capped with an orange carboy cover and standard 3-piece airlock, and has 1 oz of oak cubes in it that were previously used in the first carboy. The difference is that these cubes never leave the second carboy, and the carboy is never entirely emptied or cleaned. That lets them emulate a barrel as the bacteria culture gets established on them.
The third carboy also has 1 oz of used oak cubes in it from the first batch of Flanders Red I made back in 2013. Like the second carboy, these cubes never leave the second carboy, and the carboy is never entirely emptied or cleaned. This carboy is capped with a 3-piece airlock in a drilled natural cork stopper. I keep the cork a little moist, checking it each time I top up the airlock. The moist natural cork allows just enough oxygen exchange to help develop some acetic character.
Once a year, I blend a keg of Flanders Red from the three kegs. The first carboy tends to be a bit sweeter and fruitier than the other two. The third carboy tends to be the most sour and has some acetic character. The second carboy is somewhere in between. When blending, I leave 10-20% of the beer in the third carboy, then top it up with beer from the second carboy leaving at least 10% of that beer behind. Beer from the first carboy is then used to top up the other two which results in an empty first carboy ready to be cleaned and receive the new year's batch of wort.
The idea is that the solera will develop its own sort of character or "terroir" from the bacteria culture that stays on the oak cubes in the carboys. As well, the character will evolve over time through the transferring and blending from the newer carboys to the older ones. Right now, the third carboy has a blend of beer in it from batches of wort brewed in 2013, 2014, 2015 and 2016. The second carboy has a blend of beer from 2014, 2015 and 2016. The first carboy has only beer from January of 2017. I'll add the oak around Thanksgiving, then blend a new keg and brew the next batch or wort around the new year.