I made both types of benchwork for layouts. The one problem with the solid base is reaching across it. The open frame is easier to get under and retrieve derailed trains. The solid base layout that I had lasted only a few months before I took it apart to build another. The layout I have now is a shelf design the widest part is 2 feet wide.
The way I did mine is cut out pieces of plywood in straight tracks for were the curves are I added a triangle shaped piece of plywood to were the straight tracks meet and used small pieces of plywood to attach the joints of the wood. Then all you have to do is add pieces of wood to raise the track to the height you want it. You do not have to figure out anything except were to start and how wide you want the plywood to be to hold the track.
Here are some pictures to help understand a little better.
The two straight pieces meeting on a 90 degree and the triangle to carry the track.
A peak underneath of the corner.