I just ordered a box of carto's but will try this. Actually I did a slight variation of this I think.
I had tried the straw mod but it failed miserably, guess I did it wrong, lol.
Then last week at work I took a paper clip, straightened it, then cut a small piece that was somewhat shaped like the letter "L". Dropped that into the cart and it keeps the filler from falling through (the small bent portion making the "L" is towards the atty).
Am thinking my way is kinda like the lazy man's way. Use a support to keep it from falling in and not having to trim till you get a good fit

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And +1 on the Swiss Army knife. Have one on my keychain (been there for years) and took another one that is now dedicated to my vape kit. The scissors are indispensible and highly efficient for many tasks (vaping related or not), it is small and easy to carry, plus with the "Classic" model you get tweezers and a toothpick. All tools that I have found useful for working on carts.
Just be sure to get either a Victorinox or Wenger Knife, they are the real deal.