Keep the box for your case and for your GPU. Pack those two items in their boxes. RAM/PSU/etc should be fine being stuck on the motherboard in the case. If you got a mechanical HDD, might be worth taking that out as well. And like someone else said, anything heavy should come out of the case. Be smart about the placement in the truck as well, put clothes/blankets/pillows under and around the case.

I remove the GPU because it's so heavy and will damage the slot. Then just place it behind the passenger seat in the footwell surrounded by soft stuff (pillows, blankets, towels, etc) and roll the seat back so it's snug and firmly locked in. All you're looking to do is prevent shock. Movement is totally fine, but hitting a big pothole with nothing to absorb isn't ideal.

Safely transporting your panels requires more care. Don't want any pressure or objects to get on the displays. One object slightly moving can press a bezel or rest on the panel 10 miles in and just do damage over time for the next 690.

Edit: Shit, I prob should've mentioned how to transport panels. With as much clear room as possible. Back down, face up, no point covering the screens it'll just put a very false sense of security in. I will put them in the passenger footwell and on the passenger seat. If I have "plenty of room" to fit another box, I do not. They need that room. They are expensive, they are fragile, they are critical. They're like massive exposed motherboards that cost a lot more qnd don't have a PC case protecting them.

Remove your GPU, and basically anything that puts weight on the board. With a heavy enough air cooler, if you didn’t want to take it off, I’d probably try to lie it down flat and make sure it’s on something sturdy but soft like a couple towels. That would just mean that it’s not hanging the weight off the board.

I’m not entirely sure about glass sides, if you have it, but I’d take the panel off and keep it safe like you would a mirror. It should be fine either way but if the case flexes funny I dunno how that would play out. If you can’t make it work then it’s probably fine but that might be the best bet.

Remove the GPU, and if you have a large tower cooler the cooler and wrap them up in bubble wrap and package them separately.

If it will fit nicely wrap up the GPU and just stick it inside the case. Just make sure that it cannot move at all.

I moved over 4000 miles away with overseas shipping on a pallet that traveled about half the distance in a truck by land.

Here is how I did it:

GPU in its original box since that was made for sea shipping.

HDDs taken out, wrapped in foam, and put in boxes.

Other components stayed in the case, but the case was filled with large air pocket packing material to support the air cooling block. Computer was then put in the original case box and stored with the orientation of the cooling block vertical. The side to side movement is eliminated with the packing material and less stress was placed. Safer yet would be to remove the cooler if it is air.

If you have a glass side panel, make sure it is surrounded by packing material.

That is it! Everything arrived in perfect condition except for some books which got wet from the humid saltwater air in an enclosed space.

If you have a dGPU i would save the packaging it came in and reuse it. They are heavy enough to damage a pcie slot. Other than that its the usual soft packaging inside hard packaging. Put a seatbelt on it if you got the space in the backseat.

Size matters. I built a small form factor pc exactly because I knew I would probably move it a lot. I find that 10-15 L cases can be fit into a large backpack with some care, and 8 L cases can be fit into backpacks relatively easily.

You probably won't need it to fit into a backpack, but regardless, you will probably want to remove any unnecessary space to make carrying and transport easier. For what it's worth, I believe 15 L cases can generally fit into luggages, and I would be more inclined to trust that a PC is undamaged if left in a luggage. Especially if the remaining space is well padded