The following is my new way of constructing the trays, but keeping a similar look to what was done previously. Below you see that I laid out the tray dimensions using a marker and my steel carpenter's square. I'm building two at once here for my 6-wide Chaos Knights units.
Nowadays, I just use strips on the front and sides, leaving the back open for the expansion of the unit or addition of a character model. I also build the trays slightly wider than the bases usually one or three tenths of an inch wider to help fit in that odd conversion, a bit of slop if you will. If you have messed around a bit with GW's modular trays, you'll note a bit of play in them also. Using the thick plasticard and strips you can also add magnets to them. On the pictured VC Bashin' Orcs tray in the previous post, the commissioner instructed me to use GF9 bases, so that tray has magnetic sheet in the tray to work with the GF9 bases, as does my Knights Hospitaller unit.
So now we have the below, the general layout of what the trays will look like. The 0.2" lines are the general guidelines for the strips that will come later. The 0.3" is the outside of the tray. I normally make that a bit larger nowadays (out to about 0.4-0.5"), but with these I'm trying to somewhat match what I had done previously.
If you have not worked with plasticard before, all that needs be done with it to cut it to shape, is to score the cut line with your razor knife. Here I use the steel edge of my carpenter's square as the guide for my xacto blade.
But first...I'm still attempting to mimic what I have done in the past, so I have to do a bit more work. Nice irregular, and semi-random edges.