So I got access to a real UVB meter with enough accuracy to measure my tank. UVB only. Not UVA.

*SURPRISE*

The glass in my hood is absorbing almost all of the UVB light.

It does not matter how strong of a UVB light I am running, it does not matter whether or not I block a bit of light with paper towels, UVB just won't penetrate this glass appreciably. Maybe a 5% transmission based on the difference between measuring with the hood glass versus without.

It seems that if I want the recommended amount of UVB light, I must remove that glass. I am a little concerned about the condensation on the light fixture if I do that. But it seems like there is no other way. A big plate of fused silica is impractical.

If you read up about UVB, lots of sources do mention you must remove the glass or plastic in your hood.
I just measured this and found it to be very true.