Have you read the paper on "Very Large Ferrite Loops" by Bill Bowers
and John Bryant? It may be a little hard to find, since it was
published in the 1994-95 issue of "fine tuning's Proceedings". They
constructed rods up to 16 feet long by combining shorter pieces. Among
the observations was that it was "important to bind the rods together
so that no air gap developes between the ends of individual rods."
They bought their ferrite rods from Amidon and Nebraska Surplus Sales.