This is a topic that's cropped up a number of times on the forum over the years, and is something we're all faced with at times.
Originally, radio and TV backs were made of 'Millboard' and the slots and other apertures were die cut on presses. 'Millboard' is basically layers of paper glued together, which generally has dimpled or grainy textured finish similar to that often found on vintage radio & TV back panels. It's still available and typically used for vintage care restoration for door panels and the like. However, it doesn't machine well with a router or drill bits as it's not very dense and just 'fluffs up'. It's only available in large sheets (62" x 48") and to all practical purposes is unsuitable for our needs:
https://www.woolies-trim.co.uk/category/146/millboard
If all that's required is to make and fit a back for safety reasons rather than an identical replica, then pegboard or MDF/hardboard with rows of holes drilled towards the top will meet that requirement.
If an old back is damaged and the intention is to make a replica, it can at least serve as a pattern for a replacement, but if it's missing altogether, as they so often are, the first hurdle is to find a pattern.
Most of the sets I restore are of little value and many have had missing backs.
I've made quite a lot of replica backs for my restorations over the years using a router jig to cut the slots, and have used both 3mm Hardboard and 3mm MDF, both of which work well with drills and routers, but on balance, I have a slight preference for MDF. The terms 'Hardboard' and 'MDF (Medium Density Fibreboard) might suggest that hardboard in more dense than MDF, but it isn't.
When painted with matt emulsion in a suitable shade of brown (or aerosol sprays for that matter), and a label created, a passable replica can be achieved. It depends on what equipment and skills you have, how authentic you wish the panel to be, and how much time you are prepared to expend. Realistically, to make a replica with lots of slots can only be achieved either with a DIY router jig or CNC router. That said, there have been some stunning examples of exact replica backs made only by using drill bits and chisels, notably by forum member Robert Darwent. The '444 People's Set' for example, which has 13 sets of 61 holes (total 793) plus 15 larger diameter holes. Also, exact replica backs for round Ekcos with drills and chisels.
Pic 1: A selection of backs I made some time ago on a home-made router jig, ready for painting and labelling.
Pic 2: The same backs after painting and making/fitting labels.
Pic3: A Little Maestro damaged back with the replacement I made.
Pic 4: A replica back for a Unitra 'Figaro Special' which only called for a drill, so was very simple.
Hope that's of help and interest.