Excellent point.
A sock full of lose change might be ideal and about the most weight you could pack into a small container.
The main thing to sort out with the modeling is the basic shape and construction method, there is no great way to truly load one.
One fair warning and general problem with bridge prototypes is that they are 'one and done'. You cut it, sew it up, and hang it. If you want to tweak the design, you have to start from scratch with a new one. But if you're 'close' I suggest giving it a week or two of sleep to see if any problems develop before you jump right in to the next one.
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