The problem with roadside mailboxes is that they are usually built on public rightaway. Building a immovable structure on public property that caused injury or death could end in a legal nightmare.
The problem with the valve on the vent is that there is no visual indication as to what position the valve is in. Failure to open the valve could lead to excessive pressure build up causing hydraulic component damage.
Impluse sprinklers have a minimum pressure and flow needed to operate properly. Ha the same problem. I have both city and well water available my impluse sprinklers would not function properly with the well, when switched to the cityhose bib the sprinklers worked as they sould.
I have made inserts for two trailers I have for hauling brush and leaves. The pole can telescope and winch receiver will accept either manual or electric winches.
On hard surfaces after first pass to get the majority of the leaves make a second pass with the mower deck shut off. The spinning blades cause the leaves to blow out the side on hard pavement.