I have actually made some progress with my own, using GIMP freeware.
It happens that I have a few sheets of transparent decal paper from when I needed to replace some yellowed German cross decals on an aircraft model. As has been noted, you can't print white, so for the plane, I tried printing white cross shapes on white decal paper, and layering the black bits on top. It didn't work, because white decal paper is like the skin on milk and hopeless to work with. But I found that by masking and painting the white background, I was able to print off the black bits and layer it on top.
So first I scanned in some Airfix hussar horses. There are three horse poses and one wounded horse, which means there are seven different shabraque shapes to make, two per one and one for the one lying down. No, they're not – as you might hope – all the same shape.
Having scanned the horses in, I cut out the shabraque shape, coloured it dark blue, and then snipped zigzags off the edges. From a militaria site, I found good photos of an extant 1800 – 1820 shabraque with a GR cipher. The shabraque was red so I fiddled with that to get rid of it, then cut out, resized and pasted the cipher into either corner of each shabraque.
Here's what that looks like.
I printed them on paper a few times to make sure the size was right and when I've done all seven I'll print them on clear decal paper. I will need to go round each closely with a knife to make sure they're no bigger than the shabraque they're going onto.
The beauty of this is that the printer won't print the white. So those bits will be clear, meaning if I paint the shabraque first I should get a nice crisp white cipher.