What is the singular possessive form of trellis?

There are two accepted forms for possessive singular nouns ending in s:

Add an apostrophe (') after the existing s at the end of the Word: trellis'

Add an apostrophe s ('s) after the existing s at the end of the Word: trellis's

Example:

All the the trellis' prices were too high so I bought a strip of wire Fencing.

All the the trellis's prices were too high so I bought a strip of wire fencing.