I agree that the empty HTML field is probably the easiest way. Part of the reason is that any CSS modifications could have adverse effects on how the form behaves in mobile views and such.

The reason float doesn't work is because the modern layout uses flex containers, which make pages a lot more responsive, but that does mean adjusting the style requires a different approach.

Normally you can use justify-content to align items within a flex container, but that gets a bit tricky because the custom CSS classes don't apply to the top-level parent making it harder to target a specific one.

Instead, you can customize the field width and use margins to adjust the position of the field within the parent flex container.

For example, if you really want to do this with CSS you could add a CSS class to your field, and do something like the following

.rightAlign {
  width: 50%;
  margin-left: auto;
}