Once the 2x2 base has been attached to the plywood box, we'll cover the base with trim. Using a compound miter saw, place one piece of full 1x4 stock on edge (flat against the back fence) and make opposing 45-degree cuts with the short side measuring just over 48" in length (the long side should end up a little over 49-1/2"). This piece will be attached on one side along the base with finish nails. Use a piece of scrap cut at 45-degrees to align one end of the trim board appropriately, and the other end should be just a hair long. Mark the finish length on the other end of the trim board with a pencil and cut that end to length. Return to the assembly and tack the board into place with finish nails.
Next, cut one end piece of base trim and tack in place in the same manner, cutting one end to match the previous piece of trim, then marking the opposite end. Nail this piece into place and work your way around the base until you've trimmed all four sides of the base.
We move on to trim the top side of the box next. Using one of the 1-3/4" pieces, cut and attach a trim piece flat on the side of the box with the long edge flush with the top of the box. Continue all the way around the box, trimming the top edge of all four sides.
On to the top of the box: measure one edge of the top of the box, from the outside of one end trim board to the opposite outside trim board (the distance should be about 49-1/2").
Instead of cutting 45-degree miters with the face of the trim board against the fence of the compound miter saw, this time we'll cut it with the face down on the table of the miter saw. The piece should be cut so that the mitered angles match the mitered angles of the pieces we installed in the previous paragraph. Cut and tack the trim board into place, then continue around the top, installing top trim on all four edges.