Any sturdy work surface will suffice as a benchtop to mount the H-Series machine. You can find the mounting hole dimensions here, and generally use the bolts that the machine were shipped with. If longer bolts are required, the thread is M8x1.25mm.
For a custom bench, the two files below show the workbench design that we used for the H3 prototype machines. It is a simple design with aluminum extrusion corners, ½” and ¾” black melamine panels, and penetrations for mounting screws and maintenance access. It is a good design if you prefer woodworking tools and aluminum extrusion. For the Hybrid H3 machine, we used this same design, but added a ¼” steel plate to the top for added stiffness. A second design made with welded tubing is also shown below. This is our preferred design for the H4 machines as it is stiffer and has lower material cost. If you're not a welder, any local welding shop will be able to fabricate this quickly for you.
Note, the depth of both of these designs is the minimum required to allow use of the build enclosure. If your machine will be placed against a wall, you will want to increase the distance from the back bolt hole toward the back side.