It's been done before. It's not overly safe and it has limited fixed widths of cut. Made from an off cut of 18mm chipboard floor it's cheap though

Supplies

Off cut of 18mm chipboard, chipboard screws, 4 machine screws with lock washers and nuts

Step 1: The Fence

A basic L section held together with 4 chipboard screws countersunk so the heads didn't interfere with the running. Nothing like square but it is straight. This turns out to be ok as being parallel to the saw blade and the blade being square to the table are more important than the squareness of the fence

Step 2: Table

A 600x600mm section of 18mm chipboard flooring

Step 3: Circular Saw Modifications and Atchment

The sole plate had to be removed from the saw. This simply required the removal of the pivot bolt and the depth stop slide nut. 5mm holes were then drilled 2 per long side of the plate.
A 5mm hole was then drilled near the edge of the board (in hindsight sight it should have been further across to give more support to the uncut board when in use.) This was countersunk and a 5mm screw with a lock washer and nut used to hold the sole plate in place it was positioned roughly parallel to the edge of the board and the next hole was drilled through the board, using the hole drilled in the sole plate as a guide. This was again countersunk and a screw fitted this was repeated with the other 2 holes

Step 4: Initial Blade Slot and Fence Positioning

The riving knife has to be temporarily removed from the saw, the guard was retracted, the pivot bolt and depth adjuster refitted. The blade was set to 90° on the mitre adjustment. With the depth adjuster only just gripping the table was located so that the blade would be over the gap in the middle of the workmate, the saw switch.ed on and plunged through the table.
Now switched off and unplugged the depth was locked at maximum. The table was turned over and the squarness of the blade to the table checked.
A straight edge was placed in contact with the blade on what would be the fence side and a line drawn along it. A set square was then used to mark two lines at 90° to the axis of the blade the positions for the fence were marked on these and a connecting line drawn with the straight edge. The fence was then positioned and secured with 4 more chipboard screws

Step 5: Guard and Riving Knife

The power switch was held down with a cable tie. The assembled table saw was clamped to a portable work bench the saw plugged in and switched on(and off) using the switch on the socket(switched sockets are almost standard in the UK) a test cut was made
The edge of the cut was checked and found to be square. Whilst this is functional I decided reinstalling the saws saftey guard and refitting the riving knife was worth doing.
The saw was removed from the sole plate and the edge of the narrow side of the blade aperture marked, it was clear from looking at the blade guard this was apparent this was all that was needed on each side of the blade the distances from the blade to the ends of the guard were also measured. The sole plate and fence were then removed from the table board.
Using the blade slot as a reference the aperture for the blade guard was marked out. This could have been cut with a pad saw. Having a router I opted to use that. I set the cutter at the start point, then positioned the fence along side the router and clamped it down, I moved it to the end point checked the fence position and clamped it down, I was then able to use the fence to guide the router to cut the slot. This was repeated for the other edge and the ends cut free hand.
It was then just a case of reassembling the saw and bolting it back onto the board

Step 6: In Use

With the table clamped to a work bench and the fence secured in position it functions acceptably. I used it to remove the tongue and grove from some loft boards then re set the fence and ripped them into pannels to make draws for my work shed.