How to serve static files in Rails from a mounted path - ruby-on-rails

Question

I am trying to develop a gem to be used in Rails applications that includes serving static files (html, js, and css) at a mounted location. Using the gem's UI will be accomplished by adding the following to config/routes.rb
mount MyGem::UI.new, at: "/foo"
I have this almost working using Rack::Static, but am not attached to this solution if there is a better one.
module MyGem
module_function
def root
#root ||= Pathname(__dir__).expand_path
end
end
module MyGem
class UI
def call(env)
static_app.call(env)
end
def static_app
Rack::Static.new(nil, static_options)
end
def static_options
{urls: [""], root: asset_path, index: 'index.html'}
end
def asset_path
#root ||= MyGem.root.join("assets")
end
end
end
The files at MyGem.root/assets can be reduces to a single html file that links to a single css file in the same location using href="./index.css". (In reality they will be a React app built with homepage: "./" in package.json using create-react-app to produce ./ relative links to all assets.) I have tested this setup using both the dummy and the full-fledge React app.
This setup works if and only if I go to localhost:3000/foo/ with the trailing slash. However, if I navigate to localhost:3000/foo without the trailing slash, I successfully get the index.html page, but all of its links are broken, returning 404 not found:
GET http://localhost:3000/index.css net::ERR_ABORTED 404 (Not Found)
How can I avoid this and robustly server my static content at the mounted location, "/foo"? It's halfway acceptable as a pilot to just require the trailing slash, but is a bit dodgy to leave in a published gem.
I am open to solutions involving alternate rails routes configurations, different ways to serve the static content (contingent on being easily describable to gem users), and different ways to build the static content, and anything else.