Dean Blackketter created an add-on board which makes Teensy 4.0’s bottom-side pads available as a second row of normal pins!

It attaches underneath with castellated holes to solder to Teensy 4.0’s bottom-side pads.

If you’ve wanted a way to plug Teensy 4.0 into a sturdy through-hole socket and have access to all the signals, this looks like a really nice way to gain easy access to those bottom-side pads.

Dean shared this PCB on OSH Park and the Kicad files on Github.  Much more detail is available on Dean’s forum post and conversation which follows.