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.
If you want to retrofit an arm rest to an existing bench, take a
look at the one that Sep Tools (septools.com) sells.
It clamps to the bench & is articulated so you arm is supported all
the will it moves around.
When you get to the septools site, put ‘arm rests’ in the ‘Quick
Find’ box in the upper right corner. Then click on the picture to get
a larger view of the arm rest.
I’ve been using one of these for over 2 years & it’s great.
Usual disclaimers, just a satisfied customer.