Unbuildable Spell: Floating Disc
I'm proposing this as a Wall spell.
The disc is 3 ft wide, with a 1 inch height, making a concave surface. This is called a 'spherical cap'. The area of a spherical cap simplifies to
pi (a^{2} + h^{2})
where a is the 'base' of the cap, and h is the height of the cap. For our disc, that comes out to ~7.09 sq ft. That's...not very different from just a flat circle, but, this is ACKS. I'll be using this value to provide a relative value for the area.
Floating Disc
Wall
Area: 7 sq ft 
0.035 
Duration: 6 turns 
2 
Disc expires if forced >10' away 
0.5 
Impermeable to creatures 
25 
Range: 10' 
.5 (edit, rounding via Ary's post makes no change in ttl)

TTL 
0.3 
The area is the cost of 7/100ths of a 100' sq ft wall. The range is calculated from the difference in 0' and 30' range in Wall. The 'Disc Expires' multiplier is stolen from Illusion "Effect expires if creature moves  0.5x".
This doesn't explain anything else about the disc, however  how it carries things, how it's mobile.
After a bit of consultation with L&E, I find that a 2HD spider or beetle could be postulated that can carry 500 lbs (50 stone) at a BME of 1.68/1.62 and a CCF of .426 or .43. Let's assume we've got a spidershaped 'Unseen Servant' that carries that disc around?
Summon
Summon a Creature 
65 
Duration: Until task complete (carry the disc) 
2 
Limited to the creature type that carries discs 
.7 
2HD 
.25 
Range: 10' 
1 
Creature is limited to behavior expected of Floating Disc 
.4 
TTL 
9.1 
The duration is until the task is complete (the disc is carried until it pops), and the creature has essentially no interactive parts  it carries the disc the way it carries the disc, cannot be ordered, cannot attack, etc, etc. This is, obviously, the cheesiest of my choices, and I made this as high as possible and as limited as probable without writing a contract.
It does, however, open up the possibility of higherlevel discs, so.
This makes for a total of 9.4, rounded to 9.