You can buy a circle saw guide track but they seem to be around 100 euros. I didn’t even find one for the PKS-40 I bought. Luckily for me these things are really not hard to make. The only thing you need is a sheet of plywood or MDF/HDF that still has a factory edge. The factory edge is the edge as it was cut in the factory. It is nice and straight and if you rip a new piece of plywood, it’s at the right length for cutting sheet goods. The thickness of the material affects the depth of cut you can make and the strength. Normal 12mm (1/2 inch) or 18mm (3/4 inch) plywood will do just fine.
You start by cutting a strip of around 5cm, making sure you have the factory edge intact on one side. This strip will be our guiding strip.
After that you cut a second strip of around the width of your saw (width of base plate + width of motor). Add a few cm to that width if you want to clamp the track to your work piece (I didn’t do that because the MDF is so heavy it doesn’t really move on its own). This strip will be our bottom plate for the saw to rest on during cuts.
Next step is to glue and screw the guiding strip to the bottom plate. The factory edge should be facing inwards. Make sure the visible piece of the bottom plate is as wide as the base plate of your circular saw.
The only thing that’s left to do now is making a cut along the guiding strip. This cuts away the over-measure so that the track is custom sized to your circular saw.
Making cuts with the track is easy. Just lay out the dimensions you need on a sheet of material and place the cutting side of the track next to the line. Clamp it down of you made your track wide enough and make your cut.