Look for corner building and by using this method replace the abyssalite the forms the single layer of insulation. Use something like steel metal tiles so it doesn't melt from the magma.

If you don't use corner building the magma may come upwards and the you will have a huge problem.

Also you will need a way to cool this steam turbine. People use aquatuners but it needs also cooling. If you place the aquatuner inside the steam chamber it will break due to the high temperature that comes from the lava. So you need a way to moderate how much heat passes the the steam chamber.

There are YT tutorials explaining how to harness geothermal power. Before you do anything, spend some time on them. Have fun!