Blizzard, by your name I simply must assume you are in the "snow belt" and it is not warm right now. Well, let's try to do this so you and yours can get into hot water.
Using an 8' 2x4 you can use a level and determine how much and where you need to level. By you statement, I will assume you need to level the area from 0" to 1 3/4".
Use a table saw to "rip" a 2x4 over the length needed to achieve your 1 3/4" taper. Go to the other side of the "square" and establish your "level" and cut that taper as well. Let us assume that it too is 1 3/4". Now, connect a 2x4 to connect the two, creating what begins to be your flat level surface.
Next fill it with pea gravel, slag, sand or redi-mix and trowel it level then put a sheet of plywood base down over it, and await delivery.
You want to support the spa flat level to the ground from the center of the footwell out to the sides. Remember, you have 5,000 pounds wanting to "press down" and you need to support it "flat level". This will also accomodate the "hump" that you mentioned.
This should work, but it would be nice if it was warm outside, but it still can be done.