How can I mount a side-mount light onto a top-mount area?
You may be able to replace the mount plate and tubular arm with a custom assembly of black iron pipe and fittings that acts as a stand. From bottom up...

A 3/4&quot; or 1&quot; floor flange (whatever is large enough to cover the junction box)
A vertical pipe of ~18&quot;
A 90° elbow to horizontal
A pipe nipple of ~8&quot;
A 90° elbow to vertical (downward)
A short pipe nipple through the fixture's top cap
A nut or end cap with a hole drilled for the wires


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Be sure to protect the wiring so it isn't damaged on the pipe fitting edges. Maybe use a length of exterior extension cord between the junction box and the light housing.