This procedure turns a plot into a Polyhedron object:

pl = ListPlot3D[lowressurfaces[[2]], Mesh -> All];
cmplx = Cases[pl, _GraphicsComplex, All][[1]];
polhdr = Polyhedron[cmplx[[1]], 
  Cases[cmplx, Polygon[pts_] :> pts, All][[1]]]

This may be a start:

Map[ListPlot3D[#, Mesh -> All] &, lowressurfaces]

Hi Adam, I had previously looked at MeshRegion as described in this guide: https://reference.wolfram.com/language/guide/MeshRegions.html

Unfortunately, the constructors that use points (e.g. ConvexHullMesh) all produce surfaces that bound volumes. For example, using the data above:

s2 = ConvexHullMesh[lowressurfaces[[2]]]

Instead, I am working with surfaces - in the case of s2 the surface "above" plane #1 x #2. So I believe correct kind of Region is Polygon.

I've looked at operations on Regions in the following guide but unable to locate anything related to region cropping: https://reference.wolfram.com/language/guide/DerivedRegions.html