If you lay your arm flat on your desk with the elbow at the edge of the desk and the whole forearm supported with the palm of your hand in a mousing position, there should be a gap about three inches long and about a half an inch high from the heel of your hand along the length of your wrist where your wrist doesn't even touch the desk. ie, no pressure at all. Jamming something into that gap does then apply pressure that normally would not even be there.
I hate arm rests. They only add more pressure to your arms and encourage you to slouch and support your upper body weight on them, ultimately causing shoulder shrug and problems in the shoulders and neck. If you remove the arms from your chair and use a keyboard tray then your arms hang at a natural angle without any pressure at all and it also keeps your from slouching as much.
<< I would add a chair with a high back and head rest.