How to fill a vertical 1.5 stud wide gap?
Before I attempt to come up with an answer, let me share how I think through problems like this, as it might make it easier for others to solve problems like this in the future.
We can see that the gap itself is 30 LDU, as modules are 20 LDU, so 1.5 modules is 30 LDU. Some quick rules of thumb:

Multiples of 20 LDU can be made with regular bricks horizontally
Multiples of 8 LDU can be made with bricks, plates, and tiles vertically
Multiples of 4 LDU can be made with clever use of brackets and Erling bricks
Other numbers require the use of fairly odd pieces

We see that 30 LDU is not a multiple of 20, 8, or 4, so we'll need some special pieces for this. If we had an element that was 6 LDU thick, we could then make up the difference using 3 plates (30LDU - 6LDU = 24LDU = 8LDU * 3) or one brick vertically. Thankfully, at least one such element exists in the classic fences:

We can stack 3 plates or one brick on the center section to perfectly fill our 30 LDU gap: