@Sukera , @cjdoris , @simeonschaub , thanks a lot for your answers and sorry to answer only today! (I was away in the week-end.)
I learned a lot from your answers, and I think that I will go with @Sukera 's solution, as it lets me to keep most of my code unchanged.
Sorry to have misunderstood your idea, @cjdoris , I believed you were suggesting me to use existing primitive types rather than defining one on my own. (I was induced to believe this also because of the big warning in the documentation.)
However, I second @simeonschaub 's wish to have some way to specify padding in Julia structures: this would improve a lot the userās experience. But I fail to imagine a nice syntax for thisā¦