spiral saw query

[Glenn]

Referring to the time honored comment "When you ask a question, you get 12 answers from 10 blacksmiths, I will add the following:

Drill a series of small holes around the circumference of the circle, then pull out the center plug. Smaller the drill bit, the smaller the holes it makes, and the smoother (a relative term) the edges of the hole. Then just use a burr, rasp, or what ever to smooth up the edges or correct any clearance problems.

This method can also be used to make an angled hole through material with thickness. Just adjust adjust the angle of the drill bit to match the desired cut angle.

Put a piece of masking tape around the drill bit to use as a depth gauge.

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One thing I will mention is to waterproof everything.

OSB (fancy particle board) is particle board using big chips instead of sawdust (my opinion). I would be afraid that the even after it acclimates to the local humidity, it will do all sorts of distortions and separations if it gets wet. You may want to build a metal circular ring to firmly attach, and caulk, to the tin roof as a channel for any water to bypass the OSB. You may want to also consider keeping any inside air from getting into the roof between the tin and OSB and condensing, therefore getting the OSB wet.