Add a cleat to a piece of plywood or particle board and you can extend the surface area of your Workmate. You'll lose the clamping ability, but you'll gain a bigger surface for gluing up projects, sorting materials or even sticking the kids in the dining room on their own table at Thanksgiving.
Workmate also sells a center panel that extends your surface area but this kludge is easily made from scraps.
Another option would be to mount a tool on the spare top and save storage space in tight workshop quarters. Having a cleat would likely be irritating in this situation since it would interfere with flat storage, so a center panel may work better for, say, mounting a drill press on the table.
I like the cleat version because I'm not limited by the width of the jaws in making my top, and my jaws are usually close together, making it faster to clamp the cleat instead of a wider panel.
Step 1: Flip it
Add a cleat to underside of your top. I used glue and screws, being careful not to use too long a screw.
Step 2: Insert tab A into slot B
The cleat fits in the gap in the OEM top. You'll need to keep the cleat height short enough so that the new top will rest flat with the existing top.
Step 3: Stow n Go
Storage is easy when not in use.
I happened to have a sink cutout lying around with Formica on top. This material makes a good gluing surface. Square and true edges would be nice for using a T-Square on, so if that's important to you, buying some plywood with factory edges would be a good option.
The only problem you may run into is that sometimes it's hard to crank the jaws shut tightly on the cleat because the crank handles may hit the new top.
I learned this trick on a woodworking forum a few years ago but I don't remember who to credit. I haven't looked around lately but there are a lot of hacks floating around for the handy Workmate tool.