fli4l [1] on an ALIX board [2] (for example) is an option.
ALIX boards are reasonably energy efficient. fli4l can run from read-only media. This is no panacea (see fileless malware) but at least you can be sure that after a reboot your system is clean. Security is a primary goal [3] of the fli4l project and they maintain a public Security Archive.