If I would have known someone was going to try to build something like this, I would have added these details yesterday; The work supports do more than just keep the board at the same level as t he miter saw cutting surface. They are also designed to be used with a couple of clamps and a scrap piece of wood to create a stop block for cutting many pieces of the same length.
The legs are held on by cartage bolts with washing and wing nuts for easy removal and storage. After I made this, I realized I should have made the legs adjustable to deal with uneven ground.
Finally, here is the bench assembled. Notice the miter saw platform is actually two boxes that form a channel so the platform can be placed on the 2x6 rail. Also notice I have two specially made chocks to hold the sawhorse legs stationary to prevent accidental collapse.
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From @troyslyre