Rain Water Enclosure

Looking to have a shed like enclosure 7′ x 10′ constructed to enclose a 500 gallon rain water barrel. Construction to be 2″ x 6″, sill to have gasket and pressure treated sill material. All other can be non treated 2″ x 6″. The large wall on the left is a existing retaining wall. The concrete pad is ready to build on. Sheet metal roofing. Door opening is to be 50″ x 80″. I’m open to door construction. One standard window (32″x32″). Cabin is existing.