first of all, it is safe to open your mobile harddisk box. Actually, it is consisted of a harddisk and box with a interface circuit board. You can take the harddisk out.
I think the best way is to buy only a mobile harddisk box, then put your disk in. You should know the size of your harddisk and then try to find it somewhere, I am new in England, so cannot help you more.......
All the best!