I used to work at a place that did some custom hardware development. Usually one-off or very limited run. Any time we fabricated a case and plugged in off-the-shelf devices, certification was not necessary. If we did custom wiring we got ETL certified. I didn't run the process myself but I recall it being easy and not very costly (few thousand?) It's a barrier but a pretty low one. Our electronic work was like advanced amateur level and it still passed with minimal modifications.