Before anything else is said, I would briefly like to clarify that whilst sirens can in some cases be heard from indoors, it is very hard to ensure that this will be the case for all buildings at all times. Whether a siren can be heard from indoors depends on a variety of factors, some of which would be difficult or near impossible to combat. In short, whilst sirens can occasionally be heard indoors, generally they are more effective working as outdoor environments to alert people who are outside.
I know that there are (or were) a few people on this site that machined their own sirens, but generally I think that they were making scale models rather than their own full-scale design. Sirens chopper assemblies are pretty complex shapes, and require a lot of accuracy and precision to work. They are definitely possible, though.
Another question would be if you are planning on making this system a durable, long-term solution? If so, you will want to ensure corrosion-resistance on your sirens. Alerting Communicators or America (A.C.A.) had a useful habit of making most of the exposed parts of their sirens out of fibreglass, which obviously doesn’t rust. This allowed their models to stay in good condition for longer than some of Federal Signal’s units, most if not all of which used metal housing.
Here in the UK, we didn’t always have any type of housing on our sirens, although a fibreglass cover was an option offered. Despite the lack of housing, a lot of the existing sirens from World War II and the Cold War are still intact, although there is still clearly a difference in the condition of units with or without some form of shelter. A lot of UK units are also equipped with de-icing heaters, which may be something to think about if you get particularly cold winters (although I’m not sure you would in Paraguay).
I hope this has been useful, I’ll admit that I’m not the greatest source of knowledge for something like this, but it should give you some points to think about at least