If a bottom grate is to be installed, all base dimensions will depend upon the size of the grate.
Base attachments are made of oak; assembled with glue and screws; and attached to the 1/4"plywood base with glue and screws.
1/4" Oak plywood base was sealed with polyurethane and attached to the framing with glue, screws, and silicone based sealer to contain liquids and prevent rot.
Base framing around plywood bottom and the walkway rails are made of oak. All other wood items not already mentioned are made of pine.
Bottom grate is shown in blue and all dimensional lines are shown in red.
Top view dark lines show top walkway running between ladders at each end.