I've never seen this box but will throw a guess at it.
When you take the screw off the top to open it, the top probably sits on a plastic bar that is supported by two springs with long screws running thru them. On the bottom of the box there should be two screws that hold that assembly onto the box. Take them out and pull the assembly out so you can see the bottom of it. On the bottom of those springs there should be some screws, turn them in and it should bring the top of the box down. You'll have to check it a couple times to get it how you want it.
There's probably also a screw in the middle. That's where you adjust how soon the motor turns on so that the wheel can reach full rpm's before it hits the flywheel. You may be able to access that screw from the bottom of the box without taking things apart. But get the top to the right height first.
Again, this may not be how you're box works, but may be.