A rock through the open grill into the i3 AC Condenser radiator hard enough to puncture it, is a $1,000 plus repair, as the entire front of the car has to be dismantled to get at the condenser.
The best DIY solution is tp put some hardware cloth, or wire mesh behind the open grill, to protect the condenser, while still allowing air flow.
On the i3, easier said than done. access to the area behind the grill is almost non-existent. You cannot get at it from underneath, and the only access from above is by taking out the frunk box, then there is limited access by pushing open a flexible plastic shield an inch or so. Not ideal.
On top of that, the i3 grill is supported by double-legged 'V' shaped brackets, one leg vertical in the grill, and the other branching off behind the grill at almost a 45 degree angle. Makes mounting a mesh piece directly behind it difficult.
Here is how I did mine. Note that mine is the Rex model, which has a three tiers of openings in the grill - the BEV model has the top opening sealed.
First ordered this grill mesh - in powder coated black (they also have it in polished silver).
https://www.ebay.com/itm/CCG-UNIVERSAL- ... 2749.l2649
then created a paper pattern of the grill.
The center of the grill has support brackets exactly 6 inches apart center-to-center. I then cut a 6 inch section out of the exact center of the grill mesh. Idea is to have three grill pieces, which once assembled, have their seams hidden behind the grill support bracket.