When is the best time of year to put on propolis traps?

I have the West (I think) SHB traps which sit on the bottom board. I thought I could take off the grill and use it for a propolis trap on top of the hive. It would need to be supported, otherwise it sits on the top bars and doesn't reach fully to the edges of the hive.

Anyone tried this or know about propolis trapping?