If we assume that the resistor leads are similar to a 600 ohm
transmission line, then 1" will add enough reactance to cause
a 2 : 1 SWR on 2m. Capacitance to the aluminum box, or
an inappropriate ground path through the case, may increase
that further.
The resistor itself is rated to 3 GHz, but short lead lengths and
careful dressing of the connection to the input coax are required
to obtain the full range.
Also, make sure you don't stress the input tab. It is designed to
solder to a circuit board of the proper thickness to fit between
the tab and the ground plane.
PA0FRI has some examples
on his web site. Look under MISC
for the article on 150W dummy load.