i like magnets and some can be pretty strong for their size, but i'm not sure i understand what you're trying to do.
magnets don't like shocks so while they might be able to hold a lot of weight, i think a few bad roads could cause some trouble. i think the best way to attach something is make some discrete holes and use a couple of screws (with some clever planning you don't have to make holes anywere where they can be seen when the grillblock is of)
thinking about magnets, they would come in handy to test vortex generators. easy to relocate them.
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