Vivarium background help!

[rosie20]

Hi guys!
Yesterday I upgraded my Herman tortoise vivarium and wanted to get him a super cool background. I was looking online but they are not exactly want I want so I thought it would be a great idea to make my own. Basically I need to know a few things before I start creating and potentially harm my tortoise!

I have seen online that people have been using a spray foam but I don't want to harm him with the chemicals. Is this safe to use or is there another alternative?

Secondly what paint can I use if I can use the foam and one without any harmful chemicals too.

I really hope you can help!
Thankyou.