A fly angler's shadow box

So you tricked me adding your newest painting to your post, Clive. Very nice....it wasn't visible when I started my reply above.

Good job with the framing Clive, and your painting is beautiful.

No Helen but for anyone interested here is how I made mine...

Create a new document whatever size suits you...and then duplicate it.
In my example here I gave the background copy layer a colour to make it easier to follow what I’m saying.

Using the shape tool I drew out a rectangle shape and duplicated it 3 more times and placed them as I wanted per image 1 below.

I selected all 4 rectangle shape layers and merged them together as per image 2 layers panel.
Right click on that layer and choose simplify layer.

To create the actual cut out...

Hold down the ctrl/cmd key and click on the thumbnail of the rectangle shapes in the layers panel to bring the marching ants round the shapes and then (and this is important)
Select your background copy layer by clicking on it and then press Backspace or Delete key to create the cut out. See layers image 3.

Now either turn off your rectangle shape layer or delete it as you no longer need it. See image 4.

With your cut out layer selected you can now use whatever style you want on it, give it a drop shadow, a gradient etc;
For my final example I have exaggerated the bevel style I used to see it easier onscreen. I also used the drop shadow style and then using Patterns I picked the denim pattern to finish my shadow box. Image 5.



and my finished article...all images from Pixabay...

RE: Shadow box

I made mine a bit differently. Briefly:
• Created a blank background that was a predetermined size that would allow for 4 “same size” windows with same-size borders
• The size was in exact inches (of course, cm or cubits are also fine )
• Changed grid settings to 1 inch with no subdivisions
• Duplicated the background and, like BillieJean, made the background a different color
• Then on the dupe layer, and using the gird as a guide, cut out four rectangles of the desired size leaving a frame with four equal holes each with a 1-inch border similar to that shown in BillieJean’s upper left diagram
• Then created the shadow as per “edit layer styles”

Thanks to all. 
Clive