I mounted an AGM sideways to my frame rail of my Nissan, it was the right width to tuck in there nicely.
The enclosure is a metal electrical junction box, open top, drilled to drain if it gets wet, lined with 1/4" ply where the poles are.
The whole box sits in an "L" shaped tray framed out of angle steel welded together with some diagonal bracing on the front & back ends.
It resembles the link in the first post, but is totally enclosed. All that bolts through the fame, much easier when it is positioned with a floor jack.
It comes down every year or so to have the silt cleaned out, otherwise it stays put and does what I need without complaint.