Workshop Chinese HDMI to RF converter.
Just took a punt on one of these from AliExpress. Works really nicely, and outputs all flavours of PAL and NTSC over RF. Adjustable level of "zoom" to get rid of any under-scanning.
The power supply (5V 1A)
looks dubious, and will not be used.
One caveat. It only appears to have a VHF output... however there's a strong harmonic at UHF which is perfectly usable!
Fundamental at 67.5 ... then an output at 470MHz (Ch21) and another ch 35
-47dBm into 75Ω (61.75 dBµV) is a nice signal level for a UHF tuner. You could always wrap it in foil 😉