On the rare occasion I use the solid valve system I never have to hold it down. The weight is enough to fill on its own. I always put the yellow ring that comes with the solid valve system into the groove for the magnetic bowl so that the solid valve's weight does not deform the plastic when it is heated.
Do you mean using the solid valve bowl and bag? I use the Herbalizer bowl and solid valve bag out of fear of deforming the plastic.