How tall if the enclosure/how high is the UVB mounted above where the tortoise basks, since it looks like the UVB is mounted inside the tank?
You should also stop using that UVB bulb. https://tomaskas.co.uk/giangarden-t5ho-lamp-report/
Saw them mentioned when I was looking at water heaters a while back and was just looking for opinions. They're usually made from titanium and aren't sacrificial; they use current to prevent corrosion.
If you do use Paypal, keep in mind that some sellers will ask that you use 'friends & families' so they don't have to pay fees. This also means you can not dispute the transaction if theres a problem.
Where would this indoor enclosure be? If it's in an area thats conditioned you are probably overthinking it, but it's better than under thinking it. Radiant heat panels are prob way to go for ambient heating in indoor enclosure. If its in an unconditioned space, like garage, then what you're...
UV-open seems to be a newer term. UV transmitting might yield better results. The acrylic sheets were originally used mostly for tanning beds so searching in that direction might also help.
Example of acrylic sheet...
No, not all greenhouse materials are UV open or UV transmissive. You can get panels that are also UV open but in my experience they've usually been more expensive and harder to find. And the shipping on the panels is usually expensive because of the size.
Here's an example of a cold frame by...