Galen didn’t construct the DS, only the laser.
Then how did he construct a convenient way for the rebels to destroy it???
That’s a common misconception. He didn’t. All he did was make an unstable reactor to set off the chain reaction. That’s why he wanted the rebels to steal the plans, so they could figure out on their own how to set it off. He didn’t have a clue how the rebels would do it. It’s entirely possible he was expecting an espionage mission than a bunch of snub fighters trying to make a one in a million shot.