Hi, I had 2 sebaceous cysts (much smaller than what you describe), removed surgically from my back. The surgeon didn't drain it - he removed the entire sac intact and stitched the whole thing up.
If yours was only drained without the 'capsule' being removed, won't it fill up again? I'm sorry I cannot be of more help. But if it's any consolation, I still have a sort of depression in the skin where the bigger of the 2 cysts was removed. All the best!