I bought these bags from Wynn for my Delta 50-850 DC, but they are just a bit too small. I believe they will work well with an 18" diam DC. I think they are 4 or 6 mil thick, and seem pretty strong.
I cut one down by about 4" or so, after I stretched out the top trying to fit it on. They are [laid flat] 28" wide and 39" long - 2 pristine, and one cut a little short but otherwise good.
Wynn sells these for $13/10 plus shipping. How about $5 shipped CONUS.