You can use a rigid plug as long as it's on top of the flow mesh and is hard enough to not crush the cells created by the flow mesh and impede flow. A high durometer silicone works well.
The downside is it takes a little bit of messing around. Some moulds may be a bit too complex for it and it will cut flow, so you need to revise the intensifier design. You also need to make sure you account for the stack thickness so that the plug sits completely inside the stack layup without bridging anywhere. It can be a bit thicker than you might expect.