One key to solarisation is to not turn over the soil after cooking the top layer. If you turn the soil, you bring dormant seeds to the surface where they can once again ruin your life.
If you did that properly and still got thistles, then we're doomed!
One thing to try might be two layers of clear plastic with air trapped between - maybe 15-20cm gap. That further raises the temperature of the soil underneath. It's a method used in greenhouses too.
My soil is very good quality, having been improved with chicken & horse manure and composted chicken litter. The problem is I live on a windy hilltop and seeds from all over land where I'm planting. I've even had a wheat stalk show up.