I have an unused washing machine drain in part of my house and it's started to smell because (I assume) the water in the u-pipe has dried up. My objective is to stop the smell and I guess the only way to do this is to seal this. I want the seal to be temporary in case I want to use this again in the future. I've tried putting scotch tape over the hole but it won't stick and I'm afraid that tape in general will come loose at some point in the future.
Before I go off and buy some heavy duty duct tape or the like does anyone have some obvious answer I've missed?