special bone,
A fairly cheap, but excellent, means of being able to use a circular saw to produce clean edges (that probably won't need cleaning up with a router) is to get a length of stable wood with a straight edge (I use a 200mm wide strip of 18mm birch plywood, cut to width by my local wood supplier so that it is a reliable straight edge) and attach a piece of hardboard, or maybe even 6mm plywood, to the base of this (I used hardboard as I wasn't sure about splintering of thin plywood). Leave the hardboard protruding from the straight edge of your main piece of wood by a little more than the width of your circular saw. Run your circular against the straight edge of the main piece of wood, and this will trim the hardboard to the correct width (for that saw with that blade).
In use, mark your cut line on the door you wish to cut - I just mark it at the each end of the cut as you don't need to draw a line or mark anywhere else along the cut line. Lay the cutting guide on top, with the hardboard edge on the marks, clamp it in place, and run your saw over the hardboard, bearing against the straight edge of the 200mm piece of wood. Not only does the hardboard allow the cutting guide to be positioned quickly and accurately, it also stops any tearout on the upward face of the door.
It is also useful to use a sacrificial cutting table to lay the door upon. Adjust your circular saw so that you end up cutting into this table by the depth of one tooth of the blade. My sacrificial table is made of up of lengths of 2"x4" - two side pieces and several cross pieces screwed to those (screws are along the middle line/depth of the wood so that I can turn the table over when one side is eventually cut to shreds). The table can be whatever size you like, but mine is 6'x3'. The spaces between the cross pieces in the table make it much easier to attach clamps at many points on the table, so hold the cutting guide in place.
With these homemade guides, a sacrificial cutting table, a decent blade on my saw, and checking the saw blade for square each time before I use it, this allows me to produce very clean square cuts on plywood and MDF, so that I don't need to clean up the cut afterwards.
I made these guides, and table, after reading a very good Fine Woodworking article online, by a guy that came up with the idea after spending many years using more cumbersome methods. Unfortunately, that link is now only aceessible by subscription I think.
