For transport I worry about anything hard transferring the energy of every bump. I have always used foam inserts because it is softer and better at absorbing and knocks. You can easily make it from layers of this “cuttable” foam: https://www.ebay.co.uk/itm/404815266488?_ul=GB
It may also be possible to use something like shadow foam, but I think the foam above is better as it is softer. Once we have more of a box I think shadow foam might be the best way forward.
If it is just to keep dust off, but not for transport, there is also this printable dust cover: