Thanks for posting a photo.
Here's what I'd do, to make it simple and yet effective.
Make an "L" shaped structure, to cover the opening.
A horizontal member and a verticle member.
Leg size "L" shape to accommodate the opening.
So when it's in the closed position, there are no visible openings.
Make from 3/4" material and of suitable length to accommodate the bed frame.
Use solid wood NOT plywood.
Secure the verticle member to the horizontal, at the rear, with the vertical member resting on top of the horizontal member.
Screws: #8 1-1/2" flat head wood screws.
The screws will enter the horizontal member, from the underside, and into the verticle member.
The joined members at the rear to be flush.
Locate the screws 2'' in from each end, and additional screws spaced at 6" apart.
The 3/4" horizontal member to rest on the frame of the bed.
The vertical member to rest against the vertical section of the bed frame, when in the closed position.
The width of the horizontal and vertical members to facilitate the opening.
Use 3-1/2" brass hinges, to provide the pivoting action.
One leaf of the hinge to be mortised into the underside of the horizontal member. and
the other leaf to be mortised into the top side of the frame of the bed.
The hinge will allow pivoting of the "L" shaped structure, so as to gain access to the area, where your power cords and plugs are located.
Use 2 such hinges---each located 8" from each end.
Select hinges with removable pins. Removal pins will allow removal of the "L" shaped object, when it has to disassemble/assemble at a later date.
Make a cross-section model, an "L" shape about piece 10" long, from cardboard or poster board material,
before cutting and making the final project pieces.
Bend or form the cardboard material to make the "L".
Make the leg lengths to facilitate the structure.
Place this cardboard model on the bed frame---
use making tape to fasten one leg of the "L" to the bed frame.
Pivot the model so as to get a visual of what you want to accomplish, and as described above.
Add and access opening (hand grip) in the vertical member of the "L" shaped object.
An elongated hole 1-1/4" wide x 6" long, centered and 2" down form the top.
This hole will provide hand gripping the hinged "L" shaped member to open and close the opening, to gain behind your bed.
With this design/construction you won't have any visible screws, when the unit is in the closed position.
If you should need any additional info, please do not hesitate to contact me.