The geothermal gradient is different at different parts of the earth. You can probably bake a pizza at much shallower depths at the mid ocean ridge, near a volcano, or even at an active orogeny.

I don’t think acoustic cargo bikes are super common these days. The best two options I can think of that might fit your needs are the Tern Short Haul D8 or the BikeFriday Haul-a-Day. I’m not personally familiar with either but heard great things about BikeFriday bikes (plus I’m a sucker for folding bikes).

You could also consider something like the Crust clydesdale fork. I’ve never used it myself but heard it’s great. Only issue is that it’s usually out of stock. It may seem like a lot of money for a fork but it’s still cheaper than any other cargo option available, especially if you can find a cheap frame to use it on.