You've made biochar before? If not - it's probably helpful to do some small systems before making something large. You'll get to see how your stock of choice burns best.

We have a two-barrel system which works ok. In a few hours I'm actually about to do a second burn on our latest batch because the first one wasn't enough - I find it tricky to get a hot enough burn unless I only use tiny twigs in the retort and make sure to have a high BTU wood on the outside.

This winter I'm going to try a conical pit in the ground to see if that works better. There's a good YT video on this. Second on my list is a small mud kiln because if it works that's what I'd most like to see as a permanent feature on the land and it just seems cool. I'll put links to both of those as a reply to this post.

My favorite way to gradually accumulate a nice pile involves putting a small retort in our wood stove. The retort is just a hotel buffet pan. Works perfectly every time and we're building the fire anyway so it's almost no extra work, just part of our usual routine. In this photo, most of the wood around it has already burned down but the gases coming out of the retort are burning (it's quite hot!).