Most arm rests slide into nice holes in the bench, sort of works OK
but a bloody pain for a retrofit, worse if you walk into it.
Mine is a piece of hard wood with a 1/4" X 6" slot down the centre
sort of towards the back end. One 1/4" bolt with a wing nut allowes
the extension and angle to be adjusted. (sorry about the imperial
measurements but you can convert as well as I can
Analog/Digital Modelling & Goldsmithing