I replaced my HDD with an SSD; the SSD is thinner, so lining up the two screws at the back was tough. In the end I found some foam and cut it in to two lengths for the SSD to sit on; I used double sided tape to stick it down. After this I was able to glide the SSD into position and because it is foam it has a little bit of play that helps with reassembly. I wish I could use a material better than foam because I guess that can cause static. Fingers crossed :)
@Al Dente’s approach worked for me, but at first I didn’t understand what he meant. Basically the antenna mesh has a tongue and grove relationship with the mac mini shell; it is hard to see, but along the curved edge has a concave grove which needs to be widened. You can widen this concave grove with a paper clip and then it will fit perfectly.