Well, no wheels on the mechanical rest, but that might be next !
The rest is built from lumber that had best be 4x6 or 6x6 beams and 2x10 boards
or some laminations of 2x4s or 2x6s.
Smaller boards, siding, or luxury vinyl flooring could be used to dress it up.
Maybe a green steel roof like on Cabela's stores ?
Video does not show the recoil damper mechanism of the machine rest completely,
but you can see what looks like some cables and pulleys underneath.
Cables must attach to front of sliding gun rest.
When the slide goes rearward, wood sliding on wood, cables going out the rear of the "understructure"
might be dragging some weights along the ground on a sled, pulling them toward the gun, just a guess.