Distances:
Front of frame -- cel -- back of frame
(as small as possible)

Back of frame --- light
(at least 6-7 inches, possibly a little more)

Since the light source won't be immediately 
behind the cel (as it's not being mounted on the 
wall), I'm not overly concerned with heat & I 
think that it should be diffuse & far enough away 
that I shouldn't have problems with the 
blotchyness of the paint showing through, but I'm 
not entirely sure yet.  Well... I suppose if I'm 
going to finish this, I should probably start...