Plastic part, modification of existing Solidworks model

My company needs someone to modify an existing Solidworks model. The model is for a tungsten weight that is slightly less than 1/2 of a cylinder slice, and we need a Solidworks model that will be used for a PMMA part that is exactly 1/2 of a cylinder slice. The existing model accounts for machining loss of a tungsten tube, and we need a matching plastic part that mates with the tungsten part that is exactly the same in all respects, except it needs to be an exact 1/2 circle/tube/cylinder. The Solidworks model will help explain what we need. The modification should be quick and easy for someone proficient in Solidworks--I don't have the training to do this. Thank you.