Randomising with Constraints

OS (Win10):
PsychoPy version (2022.2.5):

Hi !

I am trying to create a number comparison task, in which 2 digits are shown side by side and the participant has to decide which is larger. I currently have 18 trials which are defined in excel. I have a loop which plays my trials in a random order. All of this is currently working well.
I would like to add six filler trials into my task which appear in a set order (1 filler every 3 trials) but I would still like the other trials to remain random.

I am quite new to PsychoPy and don’t have any experience with coding. Is this possible to do in the builder?

Many thanks!