Just take one of the various Cobweb pieces. I believe there are 'movablestatic' ones that dangle, or are corner pieces, then there are also some wall/door type ones, along with large kitpieces for making rooms that are absolutely COVERED in webs, like the spider 'boss' in Bleak Falls Barrow.
If you aren't sure what pieces to use, the best way to find out is always to look at a vanilla dungeon.