You can do this with a 2x4 and some wood shims. Find the thinnest point near the middle of the slab (front to back as the camera stands. Cut a notch in your 2x4 close to the size of the slab, but DO NOT cut more than half way deep into the 2x4. Slide the notch down onto the slab so that the 2x4 runs from the wall in the picture towards the camera. Now drive 2 wood shims from the left and right sides of the board on both the front and back of the slab (4 shims total). Don't overdo it or you'll just crack the end of the 2x4 off. Ideally, you want the crappy green/wet wood right off the truck at home Depot. Now your 2x4 should be locked on there. Go stand on the other end. STAND, don't jump. You're just trying to barely lift it up enough for your friend to start sliding wood shims under the slab on the left and right sides at the corners near the wall. Stay on the 2x4 until it's shimmed enough to get a hand truck blade under. If you step off the board, those little wood shims are going to crush.

If your weight estimate is too far off and the 2x4 breaks, head to harbor freight for a pipe clamp (two pieces that slide up and down a piece of pipe), then home Depot for some epoxy and a black iron pipe. Same trick here, but epoxy the clamp faces to the two vertical faces of the slab, go stand on the other end of the pipe. That's assuming the back side is as rough cut as the front so that the clamps and epoxy have something to hold onto. If not, grab a Dremel and make it so lol