The vacform 'coke bottle' windows of almost every resin manufacturer do not fit well. Follow the advice given above. It's good. But I use 10 minute epoxy resin. Make sure you scratch off some paint first or your seal will be less robust.
You can also cut out better shaped pieces from plastic bottles and food packaging if you can get the right bits.
You can also cut out better shaped pieces from plastic bottles and food packaging if you can get the right bits.