In class module make mBase Public instead of Private.
If in main activity you will declare Dim B4XComboBox1 As B4XComboBox, you will be able to access Left and Top of CustomView using B4XComboBox1.mbase.Left (or Top).
To access spinner (which is a child of mBase), use B4XComboBox1.mbase.GetView (0).Left (or Top).