DerbyDad03 wrote in
:

Let's say I need to extend some wires that run in the joist space below a
crawl space attic above a bathroom. The crawl space has 1 x 4 flooring.

I know I can't simply remove the floor boards, install the junction box and
then conceal it with the floorboards.

You can, if the floorboards are removable. Nail them in place, and you have a Code
violation.

However, can I replace the cut out floorboards with a piece of plywood with
an acrylic insert so that the junction box can be seen from above?

Visibility isn't the issue, it's accessibility.

The NEC says that all junction boxes must be "accessible" and defines that term as
"capable of being removed or exposed without damaging the building structure or finish or
not permanently closed in by the structure or finish of the building."

A junction box located behind a removable solid steel panel, although concealed from
sight, is still accessible and therefore Code-compliant; a box behind a clear acrylic panel
which has been cemented in place is visible but not accessible and therefore not Code-
compliant.