Planting at the bunker you have three options.
1. In the doorway(s)
2. Anywhere on the floor
3. Near the hole to the basement.
If you plant it at 1. it could look like this. Remember to plant on the right side as in the image below. Then you can snipe from T spawn (shown in second image).
(http://www.deadmen.co.uk/tactics/predator/PREDATOR_P1.JPG)
(http://www.deadmen.co.uk/tactics/predator/PREDATOR_L1.JPG)
If you plant at (2. or) 3. you can always sit in the basement or in the corner and wait for the defuse sound. The corner or just at the base of the ladder is dangerous though. The purple X marks a hole in the roof and on the overview image below you see that it is possible to throw a HE/Flash that drops down through that hole.
(http://www.deadmen.co.uk/tactics/predator/PREDATOR_CT3.JPG)
(http://www.deadmen.co.uk/tactics/predator/PREDATOR_O3.JPG)