Hi, if the matiz is similar to Lacetti then the radio has no cage, my was fitted at the sides of radio to frame work behind fascia (I ended up crow baring mine out, before realising how it was fitted, I should of removed the heating knobs and fascia surrounding radio first to gain access to brackets) on mine there was no provision to fit the cage, my new radio unit had M5 tapped holes in various places on sides in order to fit cars where the cage can't go in, I didn't want to strip down the fascia and bits, so I managed to secure the aftermarket radio cage to the storage slot below, it's not perfect but it does the job.
I'm guessing your new radio doesn't have the pre tapped holes on its sides? To do it properly you would have strip out fascia, remove existing radio and brackets, re fit brackets to sides on new radio and then fit back onto car frame.
Hope this is helpful!