I needed to define a specific area of a lake to estimate the amount of sediment in this location. I'm sure there are other use cases for this method. In this example, its in the center of a lake. Due to its shape, this would have been very difficult to select using the square "Define Map Area" box.

Step 1 - Map shows 5 areas of the lake we are looking at. To start, draw a polygon in Google Earth Pro (GE) to define the lake perimeter and save the KMZ file to your hard drive (after drawing the polygon, right click the file in GE and a popout will give you the option to Save Place As. Save this KMZ to your hard drive). Import that file into RM through the Map Boundaries box tool. Set the Lake Perimeter as a shoreline and set the depth to zero (0).
Step 2 - In GE, draw polygons to define "Exclusion Zones". These are boxes outside of the area of interest. In this case I made 2, one for the north side and one for the south side. In some scenarios you may be able to use just one or draw as many as you need to define your area.
Step 3 - Similar to step 1, save each Exclusion Zone file polygon to a KMZ file and save to your hard drive.
Step 4 - Import these Exclusion Zone files as you did for the lake perimeter. IMPORTANT - In this case, set the Exclusion Zone files to "Islands" and uncheck the "Provide Depth" box.
Step 5 - Define the map area around the area of interest. If all is done correctly, only the area you want selected should show. When you run the "Calculate Area and Volume" in the Edit tab, it should calculate only the area of interest.

For clarification, I provided some screen shots of the process. I hope this helps someone. It took some time to figure this out and would be nice if it was a feature within RM vs. going back and forth with this workaround.