I learnt this the hard way years ago. Used 4" round post one on each end and one in the middle and two plants on that run. First year they actually produced alot of fruit they pulled the post over almost pulled them slam out of the ground. Now I built them with cross ties in a "h" pattern with eyelets on both ends with his turn buckles so I can tighten them up when the need it and I used heavy high tensile electric fence wire for the single wire the runners go down. The house we bought a little over a year ago from my aunt and uncle had grapes and the made theirs out of post like a arbor and hissed field fence over the top and sides and it's a huge pain in the ass to prune. If you use the single wire type which is what my muscadine are one it's very simple and easy to prune every year