I removed all size from the boxes, so that as few assumptions are required at all.
grid-template-columns: repeat(5, 1fr);
grid-template-rows: repeat(3, 1fr);
grid-gap: 12px 4px;
The boxes take up the full space available, with the columns and rows being specified as being 5x3.
The grid-gap lets you easily adjust the size of the gap between the boxes, which results in a very flexible layout that is easy to adjust to your needs.