Hello Glenn,

It just so happens that you're able to get those last two bottom fillets to be a value of 1 and have it end up a nice watertight solid. The key is to extend the bottommost box along the long axis so you can Fillet the lines you indicated with a value of 1. Then just create the bottom box again and select a Fillet operation of 1 on the lines you indicated. Remove the internally created faces and Join the two surfaces together into a Solid. Those fillets along the top edge between the two major parts of your model will probably require a bit more thinking to blend in nicely.