What you'll need:
A cheap folding workbench (B&D Workmate or similar) - you can get them on sale at auto part stores for as low as $10. They are also comonly found in garage sales, on craigslist or even sitting in the street with a "free" sign on.
1/4 sheet (i.e. 2'x4') piece of MDF. Keep checking the scrap pile at your local home improvement store where they sell cuts with small defects at a steep discount - I paid $2 at most
A scrap of laminate flooring for the insert
scraps of wood to reinforce the table
(Optional) A router table fence
(Optional) A T-track kit
Step 1: Assemble the Table and Make the Insert Plate
Using the screws that came with your Workmate, attach the MDF to the legs.
Cut a piece of 8 mm laminate flooring large enough for the router to go through once it is attached to the plate, keeping in mind that the opening in the table is smaller than the plate's dimensions.
Trace the outline of the plate on the table.
Make a second, smaller trace perimeter for the hole.
Drill a large (~1 cm or 1/2 ") hole inside the smaller perimeter, close to its edge.
Starting at the hole, cut out the inner perimeter with a jigsaw
Using a straight router bit, practice on scraps to get the bit to the exact thickness of the laminate flooring you have.
Using clamped down "fences", hollow out the four straight lines of the outer perimeter, to the requisite depth. Finish the perimeter with the router free-hand (i.e. without a fence).
As long your perimeter is clean, there is no need for a fence to clean out any remaining extra material inside the hole.
Step 2: Reinforce the Table
Use screws and glue to attach some sturdy planks to the underside of the table to make it sturdier.
Step 3: Install a Router Fence
Fences can be very expensive; sometimes, however, they are made to fit a specific router table and when that model is discontinued, you may score a great router fence at clearance prices. If you get one, I recommend getting a T-track kit from Rockler for installation.
Step 4: Why It's Awesome
2. Takes very little space
3. Large and convenient router table.
4. Light to drag outside to on a nice day, yet sturdy enough (with ad-hoc ballast as in this picture) for just about everything - sawing, table- and above-table routing, sanding finishing, assembly/dis-assembly, etc.
5. Parts from the store-bought folding workbench you did not use (HDF planks and pegs) will serve to repair your workbench (if you bought two in he beginning and converted one of them to a router table).