Has anyone had any problem with the 3/4" plastic angle used to cover inside seals around the parameter the lower box.  The makeup of the plastic appears to grow brittle with time and cracks at the point where they are drilled and countersunk.  I've looked for plastic angle at Home Depot and Lowes, but I have not found anything.  Seems as though the cost of shipping this angle would be more than its material cost, if I ordered it from the factory.  Has anyone tried to replace it with aluminum.

Please advise,

George