I've got one of these
Its a PRESTON XS FEEDER ARM and it comes with a variety of inserts to suit many different boxes or chairs, unlike mose of the "on box" arms this one has hinged clamps that open up to go around the leg at points where you cant just slide the clamp into place.
If interested measure your legs and i'll see if there is an insert to fit, i can also take a more detailed picture to show what i mean.