I have another idea, and I have found some instructions.
You install High Sierra on an external drive and boot from that.
Read this.
Any doubts ask first.
You install High Sierra on an external drive and boot from that.
Read this.
Install macOS on an external storage device and use it as a startup disk – Apple Support (AU)
If you have an external storage device, you can set it up as a Mac startup disk.
support.apple.com