I recently saw something like those textured paints in a local DIY store (duplicolor probably) something stone effectish. I was thinking of trying it for 1/12 dashboards, but haven't bought the can yet - it's around 12€ so probably 8-10 GBP. They (dupli..) also do something like "velvet" - but I haven't seen the can in the flesh yet. For the stone types (there are several) you get an impression of the effect on the cap.
It might be the right size for your cam covers. Maybe add a generous wet coat of black or something on top to even out the dips between the sand grain like structure thingies and maybe lightly sand it again to even out the peaks, too, then follow up with another, this time light, coat of paint / colour.
May not have been Dupli-color - I need to check the pic I took on my phone, but it's something like this:
https://s-media-cache-ak0.pinimg.com...be72837caf.jpg