Yes track saw a good idea. A bit pricey though. The sawboard is a good idea too - you put it up to the marks just the same but have to clamp it. Poor man's track saw. Or track saw for someone who just needs one occasionally.
For years, I did the same thing with a circular saw and homemade sawboard. If the accuracy of the cut wasn't important, then I struck a line with a chalk line and cut free hand. It works, but it is an outside tool, I have to clean the garage when I'm done (according to my wife), and I have to observe the German mandatory quiet times for my area (1200 - 1400 during the day every day, after 2200 - 0700 Monday through Thursday, 0000 - 0700 Friday and Saturday, and all day Sunday). The resulting cut is straight, as long as I pay attention to keeping the saw pressed firmly against the sawboard and don't let it wander. If I haven't used the saw in a while, I have to calculate the kerf distance from sawboard and remember which side is the offcut. However, there is always tear out on the edge unless I remember to make the shallow reverse cut first.
The last circular saws I bought before buying the track saws were Bosch saws that were designed to work with the FSN tracks. Aside from the plume of dust and chips that went everywhere, these saws had the accuracy and ease of a track but the mess and noise limitations of the circular saws. I thought it would be a good idea to use the saw to cut a window in an interior wooden door instead of lifting the door off the hinge and make the cuts outside. I spent about 30 minutes sweeping and vacuuming the dust from that small project.
By contrast, the track saw is an inside tool, is nearly foolproof (and I know some ingenious fools who have used mine), not bound by any noise rules, the guide strip is the kerf line, and the cuts are always crisp and free from tear out.