(Forgot to mention this in the original post, sorry!)
I also thought about modifying the input table such that each of the 4 experimental groups also contains all of the fillers (A: 20 items + 20 fillers, B: 20 items + 20 fillers, C: 20 items + 20 fillers, D: 20 items + 20 fillers), but I think this also relies on a lot of copy-paste.