Joost built a Velleman K4700 Loudspeaker-protection kit in a Quad 405 with a Dada Electronics Dual Mono Power-supply.
As there is no space above the PSU he fixed it to the Back-panel behind the transformer. This may be the best solution with a classical PSU as well...
The "-" of the Speakers goes to the Power-supply and from there to the central Mass-point. Another wire from the central Mass-point goes to the Velleman.
This is not only a practical solution, it works very well!
Click on the picture to enlarge.