couple of different things so far. A 1" pretty fine polyester type, a 1.5" courser, blue material, and actually broke down and got 1 of the "zip tower" medias. So far, the white and blue are performing better then the zip media...but way too early to tell. I bought the filter in rolls from a local AC wholesaler. I can make a tower for under $5.00 on either one. Still figuring out what space to cut out of the middle of PVC. Could also use 4" PVC tube, but much harder to handle and plant...always rolling around. But much less $$, and could be held "upright" in a jig of some type without to much difficulty. Everything right now is an experiment, different GB media, different filter in verticals, comparing to traditional soil growth, etc. Keeps me out of the bars.lol

will try to get pics up

A 2-2.5" thickness would be perfect. the thickest I could find locally was 1.5. had to fold them up and make sure there was a "seam" in the middle to plants in. I used spacers to hold the towers "open" and install plants. Still determining the ideal width of the openings. The posts "close" after you cut the middle out, so it's a bit difficult to determine how much to remove. May end up using spacers of various widths to find what the best size is. Too narrow, it squashes the stems, too wide, they may slide down the tower. If I used them just for 1 crop, you could just leave the tube whole, and cut/drill holes based on that crop. I want the ability to plant multiple varieties, so the "gap" type makes more sense at the moment.