I tested a very easy to use 404 plugin by phpbits.net

A video walk through

Taking a closer look.

There are two versions. The free version at WordPress repository at https://wordpress.org/plugins/forty-four/
and a pro version at https://phpbits.net/plugin/forty-four/

After installing the plugin the settings are inside the Appearance -> 404 page

404 plugin settings

The Pro version includes a few more options:
Such as adding a logo, a menu, 3 layout options, typography, scripts and logs.

404 Pro Settings

With the pro plugin I added a logo, a menu, hide the 404 number, left the title and content as they were, and made some additional adjustments to the search bar and Go to Home page text link.

404 Pro Plugin 404 page

The 404 plugin by phpbits.net is very easy to use and has a comfortable to use UI. Check it out at https://wordpress.org/plugins/forty-four/.