partial implementation mixin class from another package?

base.go

package basepackage

type Box interface {
  Width() int
  Height() int
  Color() string
}

type BaseBox struct {
  width int
  height int
  // and a bunch more properties
  // ...
}

func (b *BaseBox) Width() int {
  return b.width
}

func (b *BaseBox) Height() int {
  retrn b.height
}

rgb.go

package otherpackage

type RGBBox struct {
  *basepackage.BaseBox
  r, g, b int
}

func (b *RGBBox) Color() string {
  return fmt.Sprintf("%s%s%s", hex(b.r), hex(b.g), hex(b.b))
}

func NewRGBBoxFromConfig(config *RGBBoxConfig) *RGBBox {
  // this is what I conceptually want to do, but it will fail
  // because BaseBox does not export these fields, possible solutions
  // below
  base := basepackage.BaseBox{
    width: config.Width,
    height: config.Height,
    // ...
  }
 
  return &RGBBox{
    BaseBox: &base,
    r: config.Red,
    g: config.Green,
    b: config.Blue,
  }
}

// Option 1:
//  copy the BaseBox implementation to otherpackge
//   - somewhat negates the point of having a partial impl, two places to make updates
// Option 2:
//  expose all the fields and give them different names to not interfere with the exposed methods
//   - the internal field names start looking silly
// Option 3:
//  create a NewBaseBox that takes as arguments a big list of fields
//   - the example above only has two, but the real code has about 8
// Option 4:
//  option 3 but using an "Options" struct with exposed fields as the argument to NewBaseBox
//   - is effectively option 2 without the weird names but with an intermediate struct duplicating
//     the internal structure

// Of these option 4 requires the smallest code change, but perhaps there is an option 5 that is more go-y?