Contestants start in their own cylindrical room--15 foot in diameter and 40 feet tall.  These rooms have an anti-magic field where no spells can be cast, no effects can be activated, permanent magic items are suppressed, potions that are draught are nullified, etc..  The walls require a DC 25 to climb.  If the user touches the ceiling they are teleported to the actual arena.   After one hour of being in the room, the participant can raise the floor by mental thought as they choose.  After two hours the floor raises until the ceiling is touched (teleporting the occupant into the arena).  Touching the ceiling determines where in the arena that person is teleported to.

The arena is circular room that is 350 foot in diameter.  In the arena, magic works as normal.  The floor is all ice (DC 15 balance to avoid slipping and falling -- pg 88 DMG), with a nonmagical fog that covers 3 feet above the ice (ie from 0 to 3 ft above the ice is fog) throughout the entire room--all vision is limited to 5 feet in the fog (although I can't remember if blind-sight is affected).  The entire area is extremely cold (see DMG pg 86 for rules).  The roof is 15 ft tall.

In the center of the room is a Mountain Dew vending machine which takes any number of coins adding up to 5 sp, dispensing a can of Mountain Dew--a noxious canned concoction which makes the imbiber immune to sleep for one hour and causes acne (alchemy, not magic).  From all the walls play "Dancing Queen" by Abba at a volume loud enough to make all listen checks -4.  To simulate this I suggest playing that song in the background during the entire battle J.   There is a disco ball above the vending machine to provide dim lighting (vision is 30 feet, below the fog is no light) though out the room.  The area above the fog is filled with various illusionary monsters dancing which randomly appear, shift, and morph.

Beneath the ice is 15 ft of clean water (albeit very cold).  The ice has a hardness of 5 with 60 hp.  The ice "regenerates" at a rate of 1 hp/round because of the extreme cold.

To visualize the arena a 350 ft by 30.5 ft cylinder.  The middle 0.5 ft is ice.  Under the ice is all water.  Above the ice for 3 ft is fog.  Mountain dew vending machine in the center.  Disco ball is the center on the roof.