NextJS, Vercel and Storybook don't work nicely together

[votemike]

Describe the bug
NextJS, Vercel and Storybook don't work nicely together when using the Next default of removing trailing slashes.
The problem is that Storybook expects a trailing slash in the URL, while Next removes trailing slashes by default. This can be flipped to require trailing slashes, but only for all URLs.
Another way around this is a custom route, but Vercel doesn't support custom routers.
While this isn't either Next's bug nor Storybook's, it'd be great if they could work together.

To Reproduce
git clone git@github.com:votemike/next-storybook-example.git && cd next-storybook-example && yarn install && yarn build-storybook && yarn dev
Then visit http://localhost:3000/styleguide and notice that Storybook does not load correctly.

System
Environment Info:

System:
OS: macOS 11.4
CPU: (4) x64 Intel(R) Core(TM) i5-5257U CPU @ 2.70GHz
Binaries:
Node: 14.16.1 - /usr/local/bin/node
Yarn: 1.22.10 - /usr/local/bin/yarn
npm: 6.14.12 - /usr/local/bin/npm
Browsers:
Chrome: 91.0.4472.114
Firefox: 70.0.1
Safari: 14.1.1
npmPackages:
@storybook/addon-actions: ^6.3.0 => 6.3.0
@storybook/addon-essentials: ^6.3.0 => 6.3.0
@storybook/addon-links: ^6.3.0 => 6.3.0
@storybook/react: ^6.3.0 => 6.3.0

Additional context
I think this could be fixed by allowing Storybook to be accessed on URLs without trailing slashes.
Maybe there is a better way.

[votemike]

Same issue in Next: vercel/next.js#26651

[ndelangen]

This is fixed in 7.0 beta