many growers build a frame for their trellis nets using pvc pipe…I am of the school that believes if you crowd your plant she will not produce the best harvest so I would suggest not putting 9 in a 5x5 tent but there are many growers that do just what you are suggesting…hope you have the grow you want
A single screen for 9 plants is a recipe for trouble: none of the plants will finish at the same time, inevitably one or more plants will need individual attention (which they can’t get if locked into a common screen). It’s always a good idea to construct a screen for each plant. That said with your plant density there is likely no room or benefit from SCROG’ing. Now if you were running two plants instead then yes to a screen. Otherwise I’d plan on simply training plants using Supercropping and topping/fimming.