According to NB it will accommodate kettles up to 19” tall. So as long as the 20gal kettle is below 19” you should be good.
Looking at the mega pot 1.2 the 20gal is 20.3” so too high. The 15gal is 18.3”. And they are 20% off right now!
I brew 10.5gal batches in 15gal kettles. Like @dannyboy58 I just watch it closely. Last week I did a 90min boil that required 14gal pre-boil. It was tight but no boil over.