Here's what I did - this will work for you.
1) Bring the pex up through 90 degree gray elec. pvc conduit "L's". You should glue conduit extensions to these so you are sure the conduit will extend well above the concrete pad surface.
2) Arrange the 4 or 6 or 8 (2 times number of circuits) side by side where you want to terminate them.
3) On the left side and right side of the group pound in 1/2" rebar as stakes - I'd probably use 3 or 4' lengths.
4) attach a horizontal piece of rebar about a foot above the foam insulation (I assume you have this in place) to the two rebar stakes you put in step 3. I attached using sturdy plastic zip ties. Wire works well too.
5) Lay another piece of rebar on the pvc conduits - this piece will be parallel to the horizontal piece in step 4). Tie each conduit to this rebar piece using zip ties or wire.
6) Tie each piece of conduit to the horizontal piece of rebar that was installed in step 4 using zip ties or wire.
7) You should now have a very stable group of conduits and rebar that will survive the pad pour.
8) After the pad has cured a few days or a week (keep it wet during the cure!), you can remove the step 4 piece of horizontal rebar. You can also hacksaw off the two step 3) vertical pieces of rebar - I've left mine in place as protection to the left and right of my conduit group.
Tom