Make the prize for Sokoban a guaranteed amulet of reflection, and guarantee a bag of holding to generate elsewhere. It could be in any of:

• On Mines’ End, not replacing the luckstone but along it.
• Somewhere below Minetown, not necessarily at Mines’ End.
• In a vault in the Dungeons of Doom, not in the first few levels.
• Somewhere at or above the depth of Mines’ End, which could be in any branch.

However, guaranteeing the existence of a reward item like the amulet of reflection risks funneling players into a single standard strategy.

Eating an amulet of reflection (which doesn’t provide any intrinsic) causes you to “reflect on your inner self”, providing enlightenment.

A cursed amulet of reflection will behave as an “amulet of absorption”: instead of reflecting rays, nearby ones that weren’t going to hit you get redirected towards you instead. Also, rays that hit an amulet of absorption wearer don’t continue past them.

Alternatively, add the amulet of absorption as its own item, and make it usually generate cursed like other “bad” amulets.