Welcome to the forums. The mounting sleeve that the head-unit comes doesn't need to be screwed into the original brackets on the car. You'll have to grind the hole slightly bigger where the stereo goes into in order to insert the sleeve, but be sure to grind small amounts of the plastic off at a time though and constantly check to see if the sleeve will fit before grinding any more off. A Dremel with the appropriate attachment is good for this.
Once the hole is big enough, the sleeve should just slide through the hole and allow you to lock it in place. Then you can insert the head-unit into the sleeve by using the DIN tools that come supplied in the box. You'll hear a 'click' sound once it's securely in place. Unfortunately I can't take a photo of mine since everything is mounted in place, but here's a photo from the net that I found as an example:
The head-unit should be nice and secure, but if there is any rattling (up or down) then pack a piece of cardboard underneath or on top of the sleeve, although it shouldn't come to that. Good luck

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