It'll be interesting to see how the Mele box will work with the Vibra fast road trans mount. I installed one and liking it so far but their site stated that it only works with the stock mount.
On the other note, if you are looking for even lighter battery at a reasonable price, I transferred the
Shorai Lithium Battery LFX36L3-BS12 Lightweight Powerful from my other project car to the Fiesta. It's a tight fit for the Mele box but it fits and weighs only about 5lbs. I had it for about a year and it survived the Phoenix area summer temperature. Not sure how it'll do in a cold climate as Lion batteries don't do well in the extreme cold, but sounds like you won't be driving the Fiesta during the winter anyways.