How hard is it to change the donut seal under rooftop AC?

Had a slight water leak in my bedroom a/c unit once and pulled the unit up and away from the roof to look at the seal. Sure enough it was crushed flat in one spot and thick everywhere else. Instead of buying the expensive factory seal I picked up a roll of pick up truck cap to bed seal, the roll was about 30 ft long and 3 inches wide for $7.00. I used that to make a new seal for the a/c unit and fixed my little leak. has never leaked again over 6 years.