Using @requires directive with federation

I have the below schema doc in my federated services
Service A schema:

type Product @key(fields: "sku") {
  sku: ID!
  size: Int
  weight: Int
}

Extending Product type in Service B as shown below works as expected.

extend type Product @key(fields: "sku") {
  sku: ID! @external
  size: Int @external
  weight: Int @external
  shippingEstimate: String @requires(fields: "size weight")
}

However, how can I do something like this:

Service B schema:

type ShippingDetails @key(fields: "id") {
  id: ID!
  product: Product!
  shippingEstimate: String @requires(fields: "product { size weight }")
}

extend type Product @key(fields: "sku") {
  sku: ID! @external
  size: Int @external
  weight: Int @external
}

Essentially, I would want to resolve the shippingEstimate in the Service B (shipping service) but, to calculate shipping estimate, the resolver requires few fields in the product type.

I am aware that the @requires directive cannot be applied on a base type as per the spec. Any suggestion on how best to achieve the above in Service B?