Yep. If the valley level isn't level with the two end peaks, adding foam or whatever will do next to nothing. If the entire surface isn't flat, foam or whatever will compress only at the peaks. This will have the effect of excessive pressure at the peaks. Ex. Two cylindrical objects laying on a floor x distance apart. Span between the two with a 2x4. Push down on the 2x4 as hard as you want. All the pressure will be on the two cylinders. There will be zero pressure on the floor(valley) between the two cylinders. Add all the foam or whatever you want to that spans between the two cylinders. Still no pressure on the floor(valley) while excessive pressure on the cylinders. Isn't this essentially the same situation as compressing Li battery cells. Massive pressure(100% of the total pressure) concentrated at both ends with zero pressure in the middle/valley.