I have been very happy with the PowerSonic AGM batteries. THey have a model with the same dimensions as the 680 and it is 18ah rated. They also have an HD version in the same size with a 20ah rating. I chose the later when I replaced it this summer. It has held up very well and only cost about $65 on Amazon.
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019