The ReTe guidelines are not overly clear re what is "slightly" vs. "very" unnatural, but the example spells seem to help.
The movement of an object across a surface is "natural" - floating in the air is "slightly unnatural", and becoming hands and grasping a person is "very unnatural". (These are all relative, of course - but an object can "naturally" be slid across a surface, can "sort of" be tossed thru the air, but cannot (easily) become hands.)
I am playing one Crippled Verditius and Corpus Deficiency, so he installed two pair of metallic wheels on his chair, and i designed one spell to make simples mechanism or clockworks works between their capabilities but under his control and direction (locks and latchs open, pulleys and hinges turn and pull, etc); Concentration and Finness are needed. I used the natural, because it's not designed to make harm (+0 like a greatest damage if you open a door against somebody). The wheels can't work against a slope or on difficult terrain.