So next up is threaded insert installation. Buy. The. Tool.
Yes, there is a tool.
Buy it. It's very helpful. Also, install them on a very sturdy surface so you can press down hard as you are screwing them in. It helps prevent the MDF from pulling up...it still does, but not as bad. Then sand it flush. I put a dot of sanding sealer in the holes before screwing them in, in hopes that it would prevent the MDF from pulling up. I'm not sure it worked.
You can also see the jig I built to help hold the full height pieces.
The secondary flare and the primary flare were put together using gorilla glue which expands, aiding in assembly even if there is a small gap.
That said, work to minimize the gap. Gorilla glue isn't all that strong, so if you have large gaps you might consider secondary blocks, to increase glue surface area...I did on mine even though my gaps were pretty small.
I held the secondary flares in place using tape, and worked very slowly to ensure that each piece was glued on as square as possible. It was probably a week between start to finish since I was only gluing 1 secondary flare piece a day, one morning one at night. Patience is rewarded here....dry fit as much as possible.