Matt,

Slot antennas began life as slots in curved surfaces, eg., aircraft
fuselages and wings. Most structural alloys will do fine. Rectangular
tubes are unusual. They are often unsightly protusions.

The maths is more complicated with curved surfaces.

But if you just copy somebody else's slot, and scale dimensions according to
frequency, the maths reduces to simple A*B/C schoolboy arithmetic.

I doubt if you are interested in an exceptionally well-defined beam. So the
differences between a rectangular and a circular tube of roughly the same
perimeter will probably be of no great consequence.

( Hi Richard - so we meet again!
----
Acqured nom-de-plume, Punchinello.)