broken 'door'

[X-ray]

If you lost the bit that broke off, then I am guessing that you broke the top 'rod' of the door. I don't reckon you could have fixed it anyway (looking at mine).
However if you have broken the bottom 'rod' off it should be easy enough to fix by inserting a replacement rod through the bottom of the frame (you have to take the whole bezel off). If you are struggling to find a replacement, just get a drill bit of the right size. You won't even have to cut it to length. Just poke the back end through the hole in the frame and into the recess in the door, use some of that heat glue to secure it and you are done. (I used a drill bit to fix a mate's hatchback. The storage space cover lost one of the retaining 'rods' and I replaced it with a drill bit).

[X-ray]

@ Orange

Ja, I was talking about the rods on the 'hinge' side.
No matter, the situation is quite good. If that was my A4000T I wouldn't try to replace the hook, I would snap it off all the way, file down the stump and have a magnetic catch. You can buy a pair of cabinet magnets for the job. On the bezel side you don't break anything off or file anything down, you place the magnet between the jaws of the catch and glue it in. The only effort will be cutting the two magnets to size (the bezel-side one will have to protrude quite a way out)
And of course you will only be able to open the door by grasping it at the top.

[X-ray]

@ Darth_X

These will be small cabinet magnets: about 3mm x 3mm x 10mm for the one closest to the drives. Even then, it sits on the outside of the bezel too far and too weak to influence a hard-drive. (On my system it would be 3 inches away from the hard-drive that is mounted in the lower-most horizontal bay).