I just had to:

• Connect the Atari antenna output to a Coaxial (F-type) to Female RCA adapter;
• Plug that in the antenna input of a the VCR;
• Use 3 RCA cables to feed the VCR A/V output into an AV-to-HDMI adapter;
• Connect the adapter HDMI output to the monitor;
• Configure the Atari to the “Channel 3” position;
• Select channel 3 on the VCR.

Easy, right? :-P Now I can play my homebrew (2048 2600) on the real thing.

By the way, if you want to buy a physical 2048 2600 cart, Good Deal Games sells them. I have no participation in that (other than coding the open-source game and pestering Jenn Harrison into drawing the cool cartridge front art), but I’m really happy to have my game on a real cart!