I would build a deck that sits flush with the top of the case bottom, and then put tubing handles on either side of the deck tall enough to just touch the inside of the lid when it is closed. Have 4 posts underneath the deck to keep it spaced off the bottom. When the case is closed, the deck will be fully supported, top and bottom. And the handles can be used to lift the deck out for maintenance. Maybe put some adhesive foam weatherstripping in the lid where the handles will make contact.
Then it's just a matter of mounting the radio in a hole cut in the deck so it can be operated comfortably with the box sitting on a desk. You could mount a cooling fan, battery, USB, chargers, voltage indicators, solar charge controller and bulkhead antenna and microphone connectors to the deck and conceal the connections underneath.
Then the lid can be used for storing cables, an HT, and if the case is big enough, a wire antenna and a folding solar panel.