" wrote:
....
I am thinking 1/2 inch copper pipe with 1/2 inch steel rod inside for
the spikes.
....
If it really has to be square, then make a tapered steel
piece and pull it through either 1/2 or 3/4 copper pipe. Obviously
this will take some experimentation. But I think it could be done.
Start with copper pipe that is longer than you need and braze on at one
end something to hold it. Use 1/4 inch steel cable and a come-a-long
to do the pulling. You may have to make several square tapered steel
pieces of increasing size and anneal the copper pipe after each pass.
....

I think pulling the pipe through two sets of rollers (one set
vertical, one set horizontal) would work better than using the
interior square mandrels you suggest.

I don't have a good size chart handy for copper pipe, but
imagine that 1/2" type M copper pipe is 0.625" OD and 0.569"
ID. So the inner periphery would be 1.788", and the outer
1.963", which would give an inner width of 0.447" and outside
width of 0.491". Half-inch rod wouldn't fit inside, but 7/16"
would. Or perhaps use 4 sets of rollers angled at 45 degree
intervals, and roll an octagonal shape with .5" steel rod
already inside.

-jiw