High terpene percentage plus any oil would work. Distillate would be ideal to keep it fairly thick so it doesn’t leak while it’s soaking but you could do a mixture of MCT + terpenes if your testing lab does not handle cannabinoids.
I’ve noticed a few of the retested carts that were clean when they were first filled but failed for lead the second time happened to be strains high in terpinolene. So I would make sure terpinolene would be one of the main constituents of the oil to intentionally assume the worst case scenario. I’ve always said terpinolene (and ocimene) are probably the harshest individual terpenes on materials. So the goal would be to intentionally do a number on the cart so your sample for testing comes in contact with all of the wetted areas faster.