I think there are a lot of ways to do this, but I would look for the required functions:
- secure the battery (I wouldn't trust Velcro, but then as a car guy I'm amazed at the lousy job usually done in RVs of all types of securing batteries)
- protect the floor from battery acid (so a tray, or box, or compartment lining of some sort)
- enclose the battery area (prevent fuel fumes and sparks from entering, or objects from coming in contact with the terminals)
- vent the compartment to the outside
Whatever meets those is fine, so I think it makes sense to just pick the cheapest, or most available, or the system you like to work with, as long as it meets those requirements.
If the battery has its own compartment, I don't see the need for a box, unless that's the easiest way to meet the floor protection requirement. For instance, madjacks tray/hold-down in a compartment with a vent would be nice.
If it is sharing with other stuff, then a vented box is probably easiest... just my guess. If the comparment does not have top access, a box might be awkward for installation and removal. The vented box shown would work in a shared compartment, and the since the bottom half is not much more than half the battery depth it might work okay - but a hold-down bracket is still needed inside that (and presumably bolted right through the bottom of the box and the compartment floor).
When I installed a battery in the trunk of a car, I addressed the floor stength issue (the risk of pulling the hold-down bolts through) with a steel strap below the floor, like a huge washer. A bit of thicker plywood could serve the same purpose in the trailer.