I wonder how a forstner bit would work. I knew a twist drill would be murder. Spade bits don't like pressure treated lumber, so I figured they might dull fast on urethane too, and the heat will dull the cutting edges too.
I actually think a chain saw would have cut the edge easier.
I never tried it, but logically, it seems like it would work. It would get the "chips" out fast, and the cut doesn't have to be a "fine" cut. There would be less friction on the blade due to the wide "kerf".
That's why a twist drill won't work well. The twist doesn't carry the chips out of the cutting area fast enough. With a material like urethane, the chips need to get out fast. Also, a twist drill will tend to "grab", and the friction plays a role too.
That's why I think a forstner bit would work well.
Oh, and I base some of this on my experience drilling through 1" thick solid rubber, and other thicknesses of rubber with a twist drill. You live and learn.
~Chuck