Funnily enough I was thinking about this the other day.
If you don't need to remove it all the time, a cheaper option may be to cover the hole with thermoplastic. I made a battery charger for a scoopic out of the stuff a few months ago and it's amazing. Maybe in black and perhaps with the hole blocked from inside too with something that also blacks out the light, it could be a cheaper solution. You just heat the plastic in hot water to get it flexible, then you could form it over the hole.
https://www.ebay.co.uk/itm/171841935289?hash=item280293d7b9:g:x2IAAOSwll1WxM-P