One trick you can try is to shut all the vents on top of the beacon housing except for one or two on each end above the lenses. That helps to better concentrate the heat flow out. I remember reading about that trick in some back issue of CTT or OGR; can't remember which.
I've also heard that some reproduction beacons don't work as well as originals do; I believe Dennis Walron from Just Trains mentioned this a while back.