From Ilford's PDF on FP5+, "HP5 Plus sheet film is coated on 0.180mm/7-mil
polyester base with an anti-halation backing which clears during development."
The holes in my 3010 (4x5) and 3005 (8x10) drums are visually out of round to let the chemicals flow behind. Larger sizes have ribs to keep the film away from the wall. For 7x17 I do two sheets in combined tanks or tank and extension, 2830 and 2560. Larger than this, a 3063 paper drum for up to a 20x24 sheet, the ribs make marks if film is used. Jobo Ann Arbor made an insert of mesh that solved this problem. That was lost in the change over. Many of us would like to find one of those. In my case it would allow developing three sheets of 7x17 at once instead of two.
Hope that helps.
John Powers