You most likely have several options. I have my anemometer mounted on the roof and the VP2 ISS (instrumentation, rain gage, temp, etc) mounted to the rail of my deck so my set up won't help you.
If the station is wireless, you could mount it to the fence with the masts. Not sure why a slight slope would stop you from cementing the mast into the ground. I've found the best way to do that is to find a "socket" that is just a little larger in diameter than the mast. It can be a metal pipe or PVC. Dig a hole about 30" inches deep, place the socket into the hole and pour the dry bagged concrete around the the socket. Pour water into the hole, the next day set the mast into the socket. I set the socket and concrete several inches below the normal ground level. That way if you have to move it you can just fill in with soil, plant some grass and it disappears. Also with the socket you can take the station down if there is a need.