Have a search for 'OWLS' - open wire line spreaders and look at the images for such.
The simplest can be strips with a hole near each corner but you'll have to feed the wire through them individually. The slotted types are much easier to install so if you have access to something like a scrollsaw they'd be easier to make. Even a hand coping saw can be used but a bit more laborious to cut.
There are complex formulas to what size to cut them but also practical limits. You won't go far wrong cutting them to the same sizes as your average commercial types and adjusting holes/slots according to the wire you have.
Be careful of them breaking, it'll annoy you no end and the vinyl types or even the old wooden dowelling spacers start to make much more sense.
Hope that helps.
All the best,