Overall design
There are many designs of smaller benches out there that could be converted to be easy to assemble and disassemble with minimal modification (e.g. nuts and bolts taking the place of glued joints).
In addition there are many purpose-designed knockdown benches, either for portability such as you want or to allow the bench to be broken down and stored out of the way if the woodworker isn't lucky enough to have a dedicated workspace.
Two of my favourite knockdown workbench designs that are available online are shown below. They make a great pairing when shown together as they take completely different approaches to the problem while still fulfilling the requirements:
[Source: Fine Woodworking]
[Source: Close Grain blog]
Weight and stability
Obviously with smaller workbenches the lack of mass becomes an issue in terms of how well the bench performs in use, primarily when planing which can impart surprisingly high forces through the workpiece to the table. There are two main forces to contend with, lifting from the leverage (resulting in the bench potentially being knocked completely over, as you've experienced with your folding metal work table) and plain pushing, which can make the bench 'walk' across the floor.
As already suggested in another Answer, adding ballast can help with these problems and it's the most common solution. Effectively you're making a lighter bench heavier, without it inherently being that massive. This ballast can be in the form of sandbags, large jugs of water and best of all, jugs filled with sand and water, which is significantly heavier than either material used alone.
While weights like this can work OK the system is not without its problems, for your application possibly the worst is that the weight is bulky and tiring to carry to and from your vehicle to where your bench will be installed. And anyway there's a better method: use your own bodyweight.
Your own weight will usually be considerably more than any reasonably portable ballast system and it is always available when working at the bench while needing zero additional effort to 'install' at the site.
Some bench designs have either low rails or a foot and you can step up with one foot on either to help hold the bench down and in position but having tried this it's not perfect — it's neither the best stance to be working in and it doesn't add close to your entire weight.
To make best use of your body weight you want to use all of it and you want to be able to do so without compromising on the best stance for the task at hand. Luckily there's an easy solution to this and that is to attach the legs (e.g. with steel fixing plates) to a sheet of plywood or OSB. The sheet must extend far enough out that you stand on it with both feet during regular use, so normally if it is wider to the right and projects out at the front only that will be enough (for a right-handed user).
This should completely solve any tendency of a lightweight bench to walk away from you as you plane and it's impossible for the user to knock over, even if while you're working the plane hits a knot and stops dead.
