Cover that is thinner than the 15-204

We do not have a thinner version of the 15-204, unfortunately. This is the only size made for this style.

Before we go any further, can you confirm that you have the Series 165 coil spring? The 15-204 was explicitly designed to work with that spring Series, so if you have something different then it will not work.

Can you also describe the issue in more detail? Is it that the cover is too big to install in the track, or that you can't get it into the track to begin with? I can tell you that installing this kind of cover in the window track can be a real pain, since manufacturers make these a tight fit. 

If the size of the spring is the same and you're only having trouble getting it installed, then it's very possible that the major issue you're facing right now is that it's simply just difficult. On average, I would say that replacing coil balance hardware like this is an intermediate level project. Even I have trouble with these sometimes and I've replaced about a million of them. I can't tell you how many times I've cursed in front of a customer due to a pinched thumb.

The video below may be of some help to you. It shows you how we install a much larger, but similarly designed system. If you still have trouble after that, you may want to consider contacting a local professional to do this for you.

Thanks for clarifying. In that case I think your best and perhaps only option is to just make the cutout in the window track wider so that it's easier. I have to stress that this is not an easy project. It's frustrating and, at times, will seem like it's impossible. If you aren't able to adapt your window to make it a little easier then you may want the help of a local professional.