I'm aging mine in my 1/6bbl sanke kegs (adapted) with solid caps over the tops.
I make sure to always have one empty on hand, in order to transfer from one to another.
I'm also using a CO2 push to transfer the batches (I do that for everything I ferment). Simply put some CO2 into destination keg, top off the source keg, vent pressure in both, add modified TC caps (to send/receive between kegs, filling to the bottom of the destination). Then give the source keg a few psi of CO2 to get the fluid moving.
BTW, I ferment mine in the same 1/6bbl kegs too. Starting off with between 4 and 4.5 gallons at the start.