Try this, take as many bathroom scales as you need to come close to what you think your weight will be. Place a brick or something similar on each one with a board across the top of the bricks. Make sure that they are all spaced the same. Then set your trailer down right in the middle of the board. Add up all the weights on the scales, subtract the weight for the bricks and board and you have your tongue weight.
Might be easier then coming up with a proper scale.