Crinkle cutter sounds like a great idea!
Another thing you can try is to cut the pumpkin and potato into a different shape. "C" or a crescent moon shape means they can grasp more easily than a straight stick because the curve gives them something to grip on. Similarly with other foods, creating a grip area is helpful, so past twists are easier than penne tubes for instance.
With some fruit and vegetables which were slippery I used to just give each baton a little squish in one place making a grip point - kind of hard to explain but it's the straight edges which are more slippery.
Some people have great success in rolling slippery batons in something dry first before handing to LO, I think someone used wheat germ (?) but when I tried this I found it only successful for the first hold, after that the outer dry layer became mushy too.
You can also try introducing a fork. Mine used a fork pretty early on, a small plastic for initially and I held gently onto the back end as I held it out for him to grasp, that way as he fed himself I could pull it back a little if he aimed incorrectly.