In my industry we receive heavy weight pumps in a reinforced double thick cardboard boxes. The pumps are typically bolted to a plywood bottom. This keeps them from moving around and getting damaged in freight.
You could cut a plywood base to sit in the bottom of a heavy duty cardboard box. Set the amp in the middle and put a 2" piece of foam panel on top. Take some perforated strap and run it across the top and use shallow wood screws to pin it down to the plywood base plate. If you get it sufficiently tight and use a box with a few inches of play around the outside of your amp it should be sufficiently sturdy for transport. In my mind at least
You could cut a plywood base to sit in the bottom of a heavy duty cardboard box. Set the amp in the middle and put a 2" piece of foam panel on top. Take some perforated strap and run it across the top and use shallow wood screws to pin it down to the plywood base plate. If you get it sufficiently tight and use a box with a few inches of play around the outside of your amp it should be sufficiently sturdy for transport. In my mind at least