If you understood that circuit enough to get it to work, then allthough maybe not a leccy, you are definately an electronics enthusiast.
The system described above is in common use in motor starters, and most relays tend to be double pole so should be easy to do, in fact you can do it with a single pole if the transistor is a high enough rating.
A couple of drawbacks with your system would be that if direct sunshine fell onto the photocell it would saturate and the system wouldnt trigger if the beam was broken, you might want to put the photocell at the end of a tube so that the sun cant hit it direct, another issue would be someone shining a torch on the photocell would easily be able to overide it and walk past the beam without triggering it, a solution to this would be a toneburst system, where the laser is modulated at a set frequency, and the photocell circuit made to only respond to this.
Kinda complicates things though, and maybe not appropriate, non the less you might be interested to know.
If you made the tube over the photocell long, like 2 feet it woudl be very difficult for anyone to shine anything down it without first breaking the beam and triggering it,