The first step in this morning's task was careful measurements, then making a cardboard tray the same size as the interior of the box. I marked the interior dimensions of the box bottom in the center of a fairly stiff piece of cardboard, then added a 1/2 inch border all the way around. Flipped the piece and covered the "OFF" side with plastic wrapping tape. Now, I trimmed it to size, cut out the corner blocks then cut thru the ONE layer of cardboard at the fold points. Then I folded the sides up so the taped side was the inside of the tray and taped it to stay folded.
Tina found this moldable foam at Hobby Lobby, she had gone to get sewing stuff, but asked if I needed anything. I asked for a couple sheets of foam to build an insert base for the knife. She bought that, but also discovered this stuff.
Never worked with this foam stuff before and there were no directions, so I donned gloves and started. Nope... the soft foam stuck to the gloves like taffy. Tossed the gloves and wet my hands.

Perfect. I packed the tray right up to the top edge and re-wet my hands and smoothed the foam. It won't be seen, as it will be covered with velvet, but... I smoothed it anyway. Then I pressed the knife into the foam to get the proper shape for the velvet to sit into .... I'll let it harden, then I'll cut the tape off the corners and open up the tray and hopefully it wasn't glued to the plastic tape and the mold will pull right off. If it works, we can test fit it to the box, cut / trim until it fits. Then we'll see about the velvet getting molded and glued to the foam with excess to go up the walls and lap over.
Next up, more sanding on the box.