This is an interesting approach. The problem with most outdoor PIR sensors is too many false alerts because of brief changes in heat caused by gusts of wind, sun going across a cloud, Or even an insect crawling across the lens. So many people end up deploying three or four sensors in a zone and only counting a positive alert if they all three report motion.
But if use of the tube narrowing the detection zone to a very specific area is working for a particular use case, that makes a lot of sense. 
Some people do that indoors if they have pets, and they will mount the sensor at around shoulder height for an adult human and use a tube to limit the detection zone. So it really comes down to the details of this case.
Another benefit of the tube that you may not of been aware of when doing the design is that it should eliminate the issue of Direct sunlight crossing the lens. It should also limit the number of false alerts because of insects.
I did want to answer one question that was brought up in the original post that I don’t think anyone mentioned yet. You asked if the PIR sensors can detect through glass and the answer is that no, they cannot. Infrared does not come through glass.
If you need to detect motion through glass, then you need a different kind of technology.
For those who want to cover a broad area, such as for a security sensor in the backyard, there are detailed discussions of outdoor motion sensing in the following FAQ:
But if you just want a very narrow detection range, typically more for a home automation type of use case such as having the porch lights come on as you approach the door, then the method discussed in this thread might work very well as long as the temperature doesn’t go much below freezing.
Battery operated devices do have problems as the weather gets very cold, so if you live in Minnesota or Canada you might need to look for a different kind of device.