I usually cut (as long as needed) pieces from an L shaped PVC profile. The PCB/stripboard is attached to the profile using hotglue, screws, or zipties and onto the panel using glue (hotglue or superglue depending on how how heavy the PCB is). I avoid metal L brackets because they are harder to work with using handtools and because of the hassle to have an extra layer of insulation to avoid shorting the back of the PCB. You can see an example here where I was able to mount quite a few heavy components (1000μF capacitors, regulator with heatsink, etc) onto the prototype panel with just a couple of drops of superglue here. Longer profile pieces also work nicely to provide support for thinner front panels.
So far this has worked fine for prototyping, but in the near future I will upgrade this method by using wider profile pieces in order to eliminate the use of glue. This way I can secure the L profile pieces in between the front panel and a potentiometer, jack, or switch, and onto the PCB using M3 screws and nuts.