That looks like a good fit; any chance you know if the subframe used on the EC300 is the same as the EC200? I'd like to get something for my 200, but don't know about fitment.
Any issues with it "binding" your seat or applying too much pressure to the seat bolt when "tugging"? I envision the strap pinching your seat together, possibly putting stress on that bolt.
I think the subframes should be similar but I'm not sure. The strap doesn't seem to bind the seat at all. I haven't had any problems whatsoever.
They are really helpful. I seem to use the front one a lot.