Received emonTx: plugged in; not receiving data via rtl_433

I think you are way over egging that.

First, it will use the same PS as the EmonTX (one powers the other).

A PiZero is plenty enough oomph.

Case, any local 3D printers or an Aliexpress

Add an SD card, a header (or solder the wires direct) and a few wires and you will be done.

YMMV