Like with the seat covers and almost all of the interior, it seems like the door panels were remade at some point by a handy and thrifty home rebuilder. They might have looked great when new, but seem to have shrunk a bit, and were getting warped. They also didn't have the pockets that the originals had, which would be useful.
When i bought the new foams and seat covers for the seat rebuild project, I ordered new inner panels from the same supplier so the main pieces of the interior would match. The rear side and main panels are original and still in decent shape, so i didn't see any reason to replace them.
The panel replacement is really straightforward, only really requiring a slightly large hole for the crank and lever spindles. There were holes in the right places, but they were too small to fit. But doors are a prime candidate for While You're In There syndrome, so i made some improvements while it was apart.
I had previously relined the widow channels with Velcro, but i still had some door rattles, so i adjusted and greased everything as best i could while in place. I added some rubber padding around the tension rod where the original felt was compressed, and made some adjustments for better window fit with the top.
Instead of using the metal panel clips, which i hate, i decided to attach the panels to the doors with Velcro. This helps avoid the potential for tearing out the hardboard when removing it in the future, and generally makes it far easier to open and service things in the door. It'll come in handy when i eventually repair or replace the sloppy window regulators.
Based on the experience of others online, i found some high adhesive, normal strength Velcro, 1" wide. The industrial version that's more common can apparently pull off of the fiberboard, and it doesn't take super grip to hold a panel in place.
I laid out basically as much Velcro as i could along the door where it would hit the panel, and mirrored that on the panel. I left a gap around the mounting holes for a door handle in case i or a future owner wants to add one. The corners especially need as much area as you can fit.
This version of the tape is incredibly sticky on the back, and will easily peel off a layer of hardboard if you have to remove a mistake. I guess this is essentially a permanent installation.
The Velcro provides a good amount of grip for the panel, and especially with the pocket edge screws in place, is completely stable. The pocket is really tight, so it barely fits a cell phone, but useful for maps (if i ever use a map again). These panels are also quite flat where the old ones were pretty puffy, so the interior feels a bit more open as well as looking tidier.