Attaching a mailbox to a railing brings back a bit of old-fashioned homesteading with a tiny touch that's often missing with congregated mailboxes with slots in the front doors. Mounting the mailbox can be a problem,however. Going through the base of the mailbox to mount it in place is difficult due to the small area to work with inside the mailbox. Coming up from beneath is not a good option due to the chance of injury from the end of screws coming through the thin mailbox base. With a couple of "L" brackets however, you can mount the mailbox using an adhesive that will hold it in place while the brackets secure it to the railings.
Position your mailbox on the railing precisely where you wish to attach it. Mark its location on the railing with pencil marks on the rail at both sides of the mailbox. Remove the mailbox from the railing.
Place a pair of "L" brackets on the railing so the brackets sit within the penciled lines, with the short rear of the "L" containing screw holes overlapping the edge of the railing at the rear and facing away from where the mailbox will open. Place the long ends of the "L" shape flat against the top of the railing. Hold the brackets in place, and then mark the location of the screw holes against rear edge of the railing with the pencil.
Secure the "L" brackets to the railing using wood screws through the screw holes in the short end of the bracket and into the rear of the railing.
Mix a batch of welding glue in a small cup using a wooden stir stick. Follow the mixing ratios on the package. Brush the welding glue along the long ends of the brackets on the railing tops with a paintbrush. Take care not to brush glue onto the railing itself.
Press the mailbox back into place on top of the "L" brackets containing the glue. Hold in place 30 seconds to begin the bonding process. Stack three or four bricks inside the mailbox to add weight to keep the box tight against the brackets. Wait 15 hours for the glue to cure.
Remove the bricks. Remove the screws holding the mailbox in place to slide the mailbox off the railing. The box can now be used.