Like has been said I would use opal white perspex.
Cover the Perspex in oracal primrose yellow, or any translucent yellow vinyl. (Saying oracal because I know you use it.) Then using the size of the pictures you will mount onto the Perspex, cut away squares from the yellow vinyl. Maybe leaving a small hairline of space from vinyl to picture leaving a neat white border to illuminate.
Better this than an ugly dark overlap.
Illuminating a recess so shallow isn’t the easiest… like Andrew said… I would either paint it white or maybe get some cheap mirror acrylic and put this in at the back first. Then… call your nearest trade electrical centre. CEF do them. There is a very slim-line fluorescent tube light complete with its gear tray/workings. They are made by OSRAM and are as thin & as bright as neon. Or there about… they also do not heat up that much at all.. Price is minimal… Run one of these down each side of the recess and the mirror acrylic should spread the light pretty well. not perfect but an option.
On the other hand you could just measure the exact size and get a slimline lightbox made for you and insert it into the area available.