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.

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?

The plywood would be big enough to be used as an access panel spanning the
joist bay, the "window" would be large enough for the junction box to be
clearly visible.

If not acrylic, what about a steel grate?