I've been fretting over these items ever since the Mahdist Revolt became my period of the moment.
I realised their importance to wargaming the period & wondered how to create some. I looked at internet photos, asked questions on forums etc. The best tip I got was to use bits of actual bushes but experiments in this direction looked awful.
Finally, I decided on wire wool. I made a prototype. I used a MDF base, randomly shaped & with bevelled edges. I put some suitably teased wool on it via the hot glue gun. The base was gritted, then sprayed brown. After drying, I drybrushed the base & added lighter brown high lights to the "thorns".
I'm quite pleased by the result (NB the photo looks very grey when shades of brown are the actual colour) so I made another 3 to begin with:
I've read that Beja warriors did jump over them on occasion. Impressive & I wish my plastic warriors luck.