Parts:

ESP32C3 OLED from Aliexpress (£2.50 each) https://www.aliexpress.com/item/1005009534750048.html

SCD40 (£11 each) https://www.aliexpress.com/item/1005006682069916.html

I don't know if my ESP Home code is optimal but it works. So far, working great.

Checks the SCD40 and ESP32 internal SOC temp every 60s and reports to HA.

3D printed case is basic but friction fits together solidly enough.

ESPHome Code:

esphome:
  name: esp32c3oled1
  friendly_name: ESP32C3oled1
esp32:
  board: esp32-c3-devkitm-1
  framework:
    type: esp-idf
Enable logging
logger:
Enable Home Assistant API
api:
  encryption:
    key: "_______________________________"
ota:

platform: esphome
password: "_______________________________"

wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password
  power_save_mode: HIGH
#flash on boot and error
#status_led:
pin: GPIO8
i2c:
  sda: GPIO5
  scl: GPIO6
  scan: true
  id: bus_a
sensor:
#Internal ESP32S2 temp for diagnostics

platform: internal_temperature
name: "SOC Temp"
id: soctemp
update_interval: 60s

platform: scd4x
measurement_mode: low_power_periodic
temperature_offset: 2.5
update_interval: 60s
co2:
  name: "SCD40_CO2"
  id: co2
temperature:
  name: "SCD40_Temperature"
  id: temperature
humidity:
  name: "SCD40_Humidity"
  id: humidity

platform: absolute_humidity
name: Absolute Humidity
temperature: temperature
humidity: humidity


font:
# gfonts://family[@weight]


file: "gfonts://Roboto"
id: roboto_12
size: 12

display:
  platform: ssd1306_i2c
  model: "SSD1306 72x40"
  id: ssd1306
  address: 0x3C
  lambda: |-
    it.printf( 36, 5, id(roboto_12), TextAlign::CENTER, "Temp %.1f",id(temperature).state);
    it.printf( 36, 20, id(roboto_12), TextAlign::CENTER, "Humi %.1f",id(humidity).state);
    it.printf( 36, 34, id(roboto_12), TextAlign::CENTER, "CO2 %.0f",id(co2).state);