Hi,
Hard to say exactly without seeing your dataset, but improving accuracy usually comes down to data quality, variation, and precise annotations.
You can check out this example we have for box detection and measurement, which uses a pre-trained model from Roboflow: Box Detection+Measure
Model: https://universe.roboflow.com/are-bla-gpxot/cardboard-box-u35qd/model/1
Might give you a good reference point.