Google has updated the Inbox app for Android, with support for tablets.
While the app did work on tablets prior to this update, it did not have a dedicated Tablet UI.
Here is what the Tablet interface of the Inbox app looks like:
The update is already available at the Play Store and rolling out to all users. So hit the play store app to check for the latest version of Google Inbox.
Oh, and Inbox is no longer a Chrome browser exclusive on PCs and Macs. It now works on Mozilla Firefox and Apple Safari browsers too.
All you need to do is visit http://inbox.google.com/ from the browser of your choice, and sign in at the page with your Google Account. It still requires an account with an Inbox invite.