Since I was limited on space in the house, I built each cube one at a time. With school (night classes) and work I did not have a ton of time March through May. Additionally, it was more difficult than I thought it would be moving around the tiny room I was building in. Thankfully about halfway through May, school ended and the temperature outside got above freezing
I moved my build into the garage, and continued assembling the first cube while simultaneously starting to build the second one. I once again ran into a situation where one of the side pieces was not quite tall enough when I did a dry fit, but by now that piece had been leaning against a wall in my basement for several months, and it probably would have fit on the other cube had I not already attached the side pieces. No big deal since I plan to veneer someday, so Donny recommended some stripes to make it even. I'll eventually add those.
For the internal braces, I decided to use 2" x 2" (1.5" x 1.5" in reality) wood pieces I had laying around from another project instead of the 1" pieces (primarily because I cut all the supplied 1" pieces too short
Honestly, for me this was the hardest part of the build. The internal braces have to be wedge tight, and I discovered the large internal middle brace I installed was not perfectly straight, so I needed several different lengths that had to vary by 1-32nd of an inch or so. If you have been reading this thread thus far, you have probably ascertained that I am very much a beginner when it comes to properly measuring and cutting wood. Luckily, I had a lot of 2"x2" pieces available, so I eventually got them all in wedge tight. I would put a small amount of glue in the middle of the ends of each brace, and slide/small hammer them into place. Then, I also learned how to shoot the nail gun diagonally, so I put a few nails into each brace. Additionally, I put silicone caulking around each brace. It took much longer than I thought, but got them all securely into place.
Side Note: I messaged Donny a TON of times throughout this build with beginner questions on how to build these, and he aways responded quickly and with answers that helped get me over roadblocks I ran into (or even ones I caused myself). I would not have had the confidence to even attempt these builds if it were not for his flat packs, and support.