This was a quick little project to prepare some defences for my 28mm table:
These are paint stirrers (free from the hardware store), with 1.5" nails for posts (a few pennies each...?), and old discarded window screen cut into individual strings for the wire. This is just a representative sample, I have a bunch more.
It would be easy to go crazy with more interwoven wire, and it would look a bit better for it, but I didn't have the time or the patience. Stringing one wire past another is super fiddly, because all of those little cross-bits act like velcro on each other, and the wire doesn't slide over itself at all. Also, looking at some illustrations of actual WW2 German wire defences, I could have modelled them a lot more horizontal than vertical... similar criss-crossing shapes, but laid out a foot or so above the ground, to make it really tough to walk through. Maybe next time... I'm a long, long way from running out of window screen!
Pretty easy and relatively quick, and I think the window screen works really well for barbed wire. I used the same materials for my 15mm FoW defences, but I think it's more suited to this scale.