The trench my wife and I have dug is a rectangle 5 feet wide and 22 feet long, with the long side on the north-south axis. The plan is to put avocado trees on either side of the north end of the hugel. The avocado varieties are supposed to grow tall, but narrow. The first elderberry will then be planted on the hugel, about 6 feet in front of the avocadoes. Then 5 feet down from the elderberry comes a couple of dwarf citrus, again on either side of the hugel, then finally the second elderberry. I hope this creates a sun trap effect, with the elderberry in the understory. The elderberry should get plenty of sun, especially as the sun swings around and hits the hugel from the side in the late afternoon. I also want them to get some shade from the citrus, since the summers here routinely spend a few weeks in the 100s. When grown, the whole arrangement should also provide a nice evergreen privacy screen between the street and the house.
We've started laying the wood in and are using vertical logs for the lower half of the hugel, as you suggested. It's astonishing how much wood fits into a trench of this size. I've also now got a lot of black clay that I'm trying to figure out what to do with.