Until I feel like dropping $40ish on a Yarai mixing glass, I have to say this Pyrex beaker works rather well. The squeeze bottle on the left is filled with a 1:1 simple syrup.
I'm also about to put in an order for an oak barrel to make aged Manhattans also. I figure 2:1 Rittenhouse to Antica Formula with a healthy dose of Angostura will work rather well.
My advice with barrel aging and bitters is to go light on the bitters. You can always add more at the time you pour and I've noticed the aging seems to amplify bitters.