I understand you bought the plastic tub and as typical the sides the mold is not even . I would take four points drive two stakes in the ground at each of these 4 sides then did a little trench under your edges to the stakes. Slide a piece of pine under the lips and along the two steaks level up your edge where you want it to be then work diagonal across the tub repeating the same then do the other two sides . Once you have that height established . Pack a little sand around the edges making an impression Of you height. Remove the tub carefully so as not to disturb your packed sides. And pour the remainder with some non shrink grout this can be made very soupy in comparison to mortar or concrete and it will not shrink. It will come pretty close to self leveling to your edges you previously established. While its still wet .. you don't have a lot of time re set the tub and see how you did. If you use some 1/2 clear tubing and fill it with water you can make a water level and it will give you the hole story.