Build this planter box for your porch using the free woodworking tutorial at the link. You could adjust the height or length of your planter to suit your needs.
Porch Planter
Tag: porch planters
Hexagon Porch Planters
Build a set of three hexagon shaped porch planters. These were built using cedar and are beautiful but you could use pressure treated wood as well. Follow the free building instructions at the link.