There are a few videos on youtube showing how to make these from drainage pipes. The posters swear by them.
Never used one.
Just made my own...
There is a place on this planet for all of God's creatures, right next to my tatties and gravy!!!
I tryed one years back and never had much intrest ive found the best thing is a 3ft deep maize pit they will keep coming back to dig for weeks
Visiting in Germany some years ago we went out in the morning with a bucket or two of maize and an iron bar. Sprinkled maize around the feed areas and occasionally jammed the bar into the ground,gave it a twirl,pulled it out and poured some maize into the hole and heeled it in.
it made the boar work at a spot for a while.