The theme should be easy enough that everyone can come dressed up. People who come without costume will be allocated a length of tinfoil to be determined by formula [1] and given access to safety pins, sticky tape, string, etc :-)

[formula]
where

b  is  guest's BMI (in kg/m²)
h guest's height (in cm)
d how much alcohol the host has had (in standard drinks, 0-10)
g host-guest gender parity (1=same, -1=opposite)
k host's orientation (0-6; 0=straight, 3=bi, 6=gay)
and the result is in lineal metres (standard 30cm width).

On-line calculator

b= kg/m², h= cm, d=, g=, k= and