you are my hero.

Im on my first playthrough, so Im not going for any achievements yet, but Ill be going for these my second playthrough. Any playthrough beyond the first 2 will be die less than 5 times.

It's good. I didn't actually have that one. I found one just after the elevator, gravity, machine gun area later in the game.

That one almost drove me mad. It's going to be hard to describe but the way that worked best for me was to put both boxes together over the sliding block. I then dropped one down just out of it's range and the other as much on the tip as I could. Then I let it draw back for just a second before I hit the gravity to make them go back up. I then timed the sliding block so that I landed the right most box on top of it on the edge and as it came back I hit gravity again. 
 
I have had more success though with this method. Get the boxes side by side on the ceiling and put a small gap between them. Drop them with the right most box hitting the edge of the sliding block so that it tips over on top of the other box. Then you can use the sliding block to keep them on top of one another while you climb over them and then push them back to the gravity trigger.
 
It's tricky no matter what, good luck!

Updated with the two secret eggs I've been able to track down so far. If anyone knows the whereabouts of more, please tell me! :)

Good stuff! I was planning to jump back into Limbo with a guide to finish off the rest of the achievements. I will probably play through again to get that achievement where you can only die five times.

That's 7 eggs that are up. I have the 8th, but it's been kept private just now.

Yeah, 10 eggs have been shown how to get.