Light off a ribbon of magnesium on top the powdered aluminum/rust mixture, and the thermite will be a go. After that, it could eat through the hood of a car, then the engine block, etc.
The key word in your sentence:
MAGNESIUM
Just use potassium permanganate and glycerin in separate chambers in a plastic capsule with a soft plastic barrier between them. Surround the capsule with the thermite compound, leaving the area of the capsule which has the barrier exposed. Drop a drop of any industrial acid on the barrier, walk away. A few seconds or minutes later (determined by the plastic of the capsule and the strength of the acid), it's ignition time.
Ah, the wonders of spontaneous AND no-flame combustion.