If you have access to 3D printing (I created an angled mount for my Video Doorbell v2 at my local library), then my first thought would be to fabricate my own. I’d take a look at this topic:
I realize that you’re not looking for an angled mount for vinyl siding, but if you read the model summary on Thingiverse you see this:
I have an adjustable angle configurable parametric Fusion 360 model if you need a different angle.
If you don’t want to create your own model, then perhaps the model’s author could supply a file that would allow you to print just a non-angled extension.