I drew up a rough plan in a notebook (see pic) in which I'd have a screen on a drawer slide that would pop out of the ammo box, and a keyboard attached to the detachable lid of the box.

I bought a suitable screen on eBay, and a black project box that was large enough to contain the screen, Pi, and fit inside the ammo container. 

With a rotary tool I cut out the viewport in the project box. (See image)

The screen does not have mounting flanges so I cut two pieces of plexiglass. One piece that fit snugly in the black project box and would have the Pi and video interface mounted on one side using brass standoffs. The second plexiglass layer has a cutout the correct size for the screen to fit.

The box assembled is shown in the images with wires for power and audio running out between the ports on the Pi.

Next step was to do some tin smithing and riveting. Using a sheet of aluminium that I found amongst my grandfather's belongings after he passed, I cut out a piece and folded it using a vise, channel iron, and hammer as a makeshift metal brake The idea was to make 3 compartments inside the box. One compartment would confine the battery, one would hold my accessories, radio, etc. and the 3rd would be where I extend the USB and network ports.

I pop-riveted the sheet in place and riveted on the drawer slides. I also created a spring from a narrow strip of the aluminium that would serve as a latch to hold the screen up in place when slid out.

Using a piece of grey plexiglass I mounted the USB and network extensions and then fastened it into the rightmost slot in the aluminium divider. 

Connecting all the wires to the box and installing the inner part of the drawer slides on the project box, I then assembled it. 

Locating a keyboard was a challenge. I could find a keyboard that fit on the ammo box lid, but not one that was wired. I hesitate to use a wireless keyboard because I imagined wanting to use this to send secure encrypted messages over packet radio and if your keystrokes can be discerned at a distance by its radio emissions, it defeats the purpose. Wired keyboard switches still generate discernible radio noise, but only at very close range. I also wanted a keyboard that had a built-in pointing device. 

The keyboard I was able to source that met my needs best was too large to attach to the box lid and so it is instead, stowed in the space between the aluminium divider and the project box/screen.

A high-capacity power bank with built-in solar panel is used as a battery and fits snugly in the middle compartment. I used a piece of strong "tuck" tape to make a tag that I can pull to remove the pack from stowage.

My accessories including a Baofeng radio and a RT-SDR and thumb drives fit well in the first compartment if the Baofeng antenna is detached.