Lay a leg flat on your work surface, with the countersink side of the thru-holes down. Stick a piece of threaded rod in each hole. Take a stretcher that is marked to have one end adjoin the top of this leg, stick a dowel center in its dowel hole, line it up against the leg, using the threaded rod for positioning, You want the top of the stretcher to be even with the top of the leg, or just slightly above it. Give the end of the stretcher a whack with your rubber mallet. This will leave a mark indicating where the matching dowel hole in the leg needs to be drilled. Repeat with the lower stretcher than adjoins this leg. Then repeat for the other leg that will form this trestle, and the other ends of the two stretchers.
Flip the base upright, put the MDF on top of it, then use a straightedge to draw two straight lines joining the outside edges of the legs and extending the width of the MDF. I used the countertop as the straightedge. We cut it with our cutting guide, which is based on the factory edge of a sheet of 1/4" plywood, so it should be straight enough. Use a carpenter's square to transfer these lines onto the ends of the MDF.
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.)