Thank you! I (or @tomshwom or one of the other epic tech people here on steemit) may write an article on that in the future, but until then you could start by trying to get a standard Ubuntu image to a SD-card. That wouldn't allow for any customization, but at least it would give you a pretty safe browser environment without malware. This could be accomplished by using Etcher if you are on Windows, or dd if you use Linux. Have a look at this as a start: How to write a .iso file to an SD card?

Get back to me if it works!