To unlock the Episode VI Bonus door, you need 32 Gold Bricks.
This one will be quite difficult. Return of the Jedi has a total of four long levels out of six, which will eat up your time. In terms of getting studs, Speeder Showdown is probably the best choice, but don't spend too long getting studs, you will miss the hour limit if you do. Simply destroy things as they come to have a good chance. In terms of long and difficult stages, Jabba's Palace and The Great Pit of Carkoon come up quickly and early, and each takes about 10 minutes to complete. Add on getting unlucky in Speeder Showdown and Into the Death Star, and you will be pushing the time limit very quickly.
Location: Endor
Time Limit: 7 Minutes
This one's really easy, they're rather generous with the 7 Minute time limit. Again, Boba is the character of choice. Simply shoot and bomb things as much as possible, the studs drop in a very nice way so it's easy to pick them up after you destroy something. You could fool around for a couple minutes and still meet the quota, it really isn't that difficult.
Location: Endor
Time Limit: 5 Minutes
This one on the other hand, is quite hard. The area is quite small, and it's hard to see what is breakable and what is just normal ground. There are two torpedo targets here and they are your best friend. Waiting until you have Infinite Torpedos is probably a good idea, as you don't have to worry about getting torpedos then. Considering the small space, there will be times where there just won't be anything to shoot.