First, cut one long edge. Second, cut a short edge, making sure it's square to the long edge you just cut. Clamp both pieces of the edging you'll be using along the long edge you've cut, and measure the width of the base plus 1/4-1/2", mark that, and then lay out a line through the mark that is square to the end you've cut, then cut along the line. Finally, cut the remaining short edge square to both long edges. (The length of the top doesn't need to precisely match anything, so we don't need to bother with clamping the trim before measuring.)
I had a similar experience with running electrical power to a shed. It turned out that I could build a 12x12 shed without a building permit as long as I did not run utilities, but getting a permit opened a whole new set of requirements that forbid me from having a shed at all. And when the nosey old woman next to me kept calling the building inspector on me, it was suggested that I could run an extension cord out to the shed anytime I wanted as long as it wasn’t fastened in a permanent manner.
I made a low table out of a couple of step-stools, my hollow-core door, and one of the MDF panels that would eventually form part of my top. I was concerned that any oil that dripped on the door might interfere with its glue adhesion, when I finally get around to the project for which I'd purchased it. The top side of the top sheet of MDF, though, I planned to oil, anyway. (Ditto for the bottom side of the bottom sheet.
Post Woodworking’s Hampton shed is a major standout in design. The sleek farmers porch immediately draws the eye into its pure New England charm and provides the utility of shade and a cool place to sit and enjoy a cold one. Like all of our sheds, the six-panel doors which add to the structure’s aesthetic come standard. They’re made out of fiberglass to last through many decades out in the elements.