I've seen some with tiny pennants (flags) that stick up above the cowling so that you won't forget they're installed and takeoff. A friend of mine with an RV-6 took off with the cowl plugs installed last week and almost cooked his engine. I made cowl plugs out of a swimmer's kick board. Cost me nothing as I had the board, a rope and a "remove before flight flag." It doesn't have the tiny flags but the two plugs are connected by the rope and if I left them in, the prop turning would catch the rope and fling them off.
