Sorry for reviving this old thread, however just what I am looking for. I have just bought these batteries and have a pair so need one under the front bench.
@timster66 did you try fitting the batteries on its side? I've just measured up and it's definitely tight. I don't particularly want to cut a whole in my seat base.
I'll be making up a tray for it out of sheet metal, so I am not concerned if it doesn't sit on the floor. I was planning to have it just below the top.