I put my fold up panel on top of the folded up tent on our soft floor, in between the ground sheet and annex walls. Once the tent cover is secured its safe and sound and does not move around. Throwing it up on top during the pack up also helps me get out the remaining air in the tent due to the extra weight it makes everything easier to pack up. Another bonus is if I want to just pull the solar panel out to top up the batteries it's not a big deal to open a few bungie cords and pull it out.
Cheers,
Matt