Rock steady and portable don't usually go together but here is an idea I've seen that should get you close. The legs are taken from a folding table. You can buy them from Harbor Freight or any big box hardware supplier, or off of a throw away table. The top is made from 3/4" plywood. Cut the top any-way you want it and trim the edges or sand them smooth. The hard part is getting the legs to close. In our bench the legs will overlap when you try to close them because the top isn't long enough. To overcome that, mount one of the folding legs to the table with a 2x4 spacer between the legs and the table. You'll also need a spacer for the hinged locking leg's attachment. If you set up the table now, one end will be higher than the other by the width of the 2x4. To offset this difference, you CAREFULLY make a small cut upthe side of the feet on the other legs and insert appropriate sized pipe 2-3" in the legs and trim them to length so the table will be level. You're just trying to insert a snug pipe to level the whole thing up. I thought I had saved a set of plans on my computer but I can't find it. Maybe I printed it out and it's in one of my binders somewhere.
Hope this helps,
jd1