I'd recommend covering up this for these reasons:
D2S bulb plug is not waterproof. Water will seep in over time and 23KV start pulses may short through water and kill ballast over time.
All OEM HID projector units are made out of relatively thin plastic, making them mechanically much more frigile than any other component you have underhood.
I have seen people using
Small CV joint boots - I really like that idea
round ABS plastic end caps found at plumbing dept.
Custom ones - I very proud of this one:
(sorry for all the popups)