Ideally some hybrid design, shape/size not sure I have a definite option but I think the best barrier is something as tall as the cage BUT made out of ring ropes with netting between them.
Basically a barrier which has a lot more "give" to it than the cage to cut out cheap tatics, makes cage clinching harder, makes limiting opponents striking defence against the cage harder, makes leaning on the cage for takedown defence harder, makes using the cage to get back up harder.
All those areas are I think relatively lower skill tactics which have become massive parts of MMA and the sport would be better without them IMHO.
It would make cage grabbing fouls both less effective and much easier for the ref to see as well since the ropes/netting would be more obviously moved by them.