Cookin, if you click on this link it will give you info on a product that will do just what you are asking. It is a material that is designed to caulk large joints between concrete panels in warehouse structures. If you contact a commercial building supply house (not HD or Menards, etc) they will be able to sell a couple tubes to you in varying colors. It may not be this specific brand, but this is one of the best.
file:///C:/Users/Jim%20and%20Brenda/AppData/Local/Microsoft/Windows/INetCache/IE/IGWO6GKK/Pecora_Tilt-Seal_Brochure.pdf
You would simply clean and lightly sand your small roof surface, apply the sufficient amount of mastic and place the antenna base in it, and let it sit for approximately seven days.
Yes, this material can easily be used on a sloped surface. It is very heavy bodied and will run under no circumstances.
Probably in the $7.00 per tube range. Standard caulking gun is necessary.
This is one company:
http://www.metrosealant.com/Locations