What's the current decorator convention?

app/overrides/my_store/spree/product_decorator.rb

module MyStore::Spree::ProductDecorator
  # ...

  Spree::Product.prepend self
end

Not sure how I feel about namespacing the namespace.

if you call it

Spree::ProductDecorator

and any extension does that too, you'll collide

The original point for namespacing in a Rails app with Spree (at the time) was to help convey to developers "Hey, this is actually your Rails app, Spree is still a guest and an engine."

plus it makes it clear if you look at the ancestors when you're debugging what the source of the decorators is

In fact, there's no decorator convention anymore. Please, see https://github.com/solidusio/solidus/pull/4231. But if you need to monkey patch, yeah, using

prepend

is the less obtrusive one. However, before doing that, always consider if you can: • Use a custom service class • Use an event handler See https://edgeguides.solidus.io/customization/customizing-the-core for details