Skylight Detail

The plastic bathroom skylight on the campervan developed a tiny crack last year, allowing a little rain, and bizarrely a couple of flies, into the sealed unit. We managed to remove the whole unit last week (after partially dismantling the bathroom) in order to attempt to seal up the crack, prevent it spreading further and dry the inside out.
We had investigated obtaining a replacement window unit, but the only one we could find available was for sale in Germany for about €800, plus hefty import taxes (thanks brexit).