I have a 2015 ES and I'm installing an FZ-1 Fuzeblock. Dimensions: L 3.25" x W 2.5" x H 1.25". Here's the spot I found that should work.
Right beside the battery there's a little protrusion. It isn't used for mounting and nothing attaches behind it. Maybe it's just from the moulding process, I don't know. But I cut the little bugger off and my FZ-1 fits perfectly in there and is accessible under the first cover. I'm trying to find the clips needed to use the heated grips plug in to power it. If not I'll just run leads to the battery. I'm going to mount it with heavy duty velcro instead of trying to screw it down. That way I can leave some extra wire and lift it off if I need to. I was going to do it all yesterday, but I always glue the velcro down and wanted to give it a day to set. I'll post the finished pictures later.