You might be able to reproduce the same concept that the table with leaves has. Instead of putting the clips on the underside of the table (since that would not allow it to lay flat over the existing table) perhaps you could get smaller hooks and place it on the underside where it overhangs on the side. Would be a little more work, but definitely cheaper than a new table. 😉
This super-strong and simple-to-build workbench is may be the project you've been looking for a long time. You have to select some free workbench plans to create yourself a working table in your shed that after you can use it when you are working on your projects and maybe it can provide you some extra storage, depends upon which plan you are choosing to DIY.