either will work. Personally I prefer belly heat racks over back heat. I have found that belly heat racks run their flexwatt at a lower temperature than back heat racks have to.

Wooden shelfs are a grey area in my opinion. some can handle the flexwatts heat that the humidity well, and others do not. I would probably go with a shelf like the one in the link you posted, but that is an opinion of mine and it has no data to back it up.