I use a Thule Vectros bumper for by 2015 15" MacBook Pro and have never run into any thermal issues. However, I typically use my MBP on a stand on my desk that allows for all-around air circulation. The bumper doesn't block any vents and if you don't use a stand the height of the case and the feet raise the MBP off a desk or table to allow for some air space.
The bumper protected my MBP when I stumbled over one of the dog's toys and knocked it into the corner of a cabinet but it is not lovely to look at and was slightly fiddly to install.