Building Electron app can't find resources files - reactjs

Question

I have an electron react app (built with create-react-app) which runs fine in development mode, but when I use electron-packager to publish it both the .css and .js files have the error "Failed to load resource: net::ERR_FILE_NOT_FOUND".
Looking in the developer tools network tab I can see it is looking for the files at file:///static/js/main.83d4505c.js and file:///static/css/main.00ef8ad8.css while the file/folder structure is:
build
static
js
main.83d4505c.js
css
main.00ef8ad8.css
App-darwin-x64
App (The executable app)
public
src
...
I use the below code for my main window:
mainWindow.loadURL(
isDev
? 'http://localhost:3000'
: `file://${path.join(__dirname, '../build/index.html')}`
);
I have tried adding "homepage": "./", to the package file as suggested on https://github.com/electron/electron/issues/1769 but it doesn't help.
Package.json
{
"name": "app",
"version": "0.1.0",
"private": true,
"scripts": {
"start": "react-scripts start",
"build": "react-scripts build",
"test": "react-scripts test",
"eject": "react-scripts eject",
"electron-dev": "concurrently \"BROWSER=none yarn start\" \"wait-on http://localhost:3000 && electron .\""
},
"homepage": "./",
"main": "public/electron.js",
"directories": {
"buildResources": "build",
"app": "build"
},
"eslintConfig": {
"extends": [
"react-app",
"react-app/jest"
]
},
"browserslist": {
"production": [
">0.2%",
"not dead",
"not op_mini all"
],
"development": [
"last 1 chrome version",
"last 1 firefox version",
"last 1 safari version"
]
},
"dependencies": {
"#emotion/core": "^11.0.0",
"#emotion/react": "^11.9.3",
"#emotion/styled": "^11.9.3",
"#mui/icons-material": "^5.8.4",
"#mui/material": "^5.8.5",
"#mui/styled-engine-sc": "^5.8.0",
"#testing-library/jest-dom": "^5.14.1",
"#testing-library/react": "^13.0.0",
"#testing-library/user-event": "^13.2.1",
"chokidar": "^3.5.3",
"electron-is-dev": "^2.0.0",
"electron-reload": "^2.0.0-alpha.1",
"react": "^18.2.0",
"react-dom": "^18.2.0",
"react-scripts": "5.0.1",
"styled-components": "^5.3.5",
"web-vitals": "^2.1.0"
},
"devDependencies": {
"concurrently": "^7.2.2",
"electron": "^19.0.4",
"electron-builder": "^23.0.3",
"electron-packager": "^15.5.1",
"prettier": "1.17.0",
"wait-on": "^6.0.1"
}
}