I'm using a the original power adapter through a hole behind the Echo Show. You coud maybe use something like this.
https://www.amazon.com/dp/B00AH5XCLC/ref=cm_sw_r_cp_api_o2OcybS7JV8J3

The original power adapter provides 18V - 1.68A (30W), POE 802.3at Typ2 can provide 25.5 - 30W!