Holy cow, you did get some wild expansion.
Try it like this-
bottom up build. 2" sand, sheet metal, 2" sand,-2-3 layers carpet, rubber mulch half way full, more carpet, fill with mulch, carpet, lid, carpet carpet.
I & several others put some time into seeing what works in these. The 454308 were 2"+ away from the sand. I'd never recover any of them if I had to diggem out of the berm. Do keep sand & mulch seperate for ease of use.
AND yes, Brad's got a good one. He'd be a crotchety old fool to let her go.