I suppose you can use anything you can fit on there in the proper position. A old lid off a pail, metal or plastic. Just whatever is pleasing to you for any reason.
However if your bike has the DR limits properly adjusted and nothing else is wrong to make the chain come off the rear cassette or freewheel often, then why do you need spoke protector?
- Which seem easier for you to make fit?
- Don't know, spoke protector design and engineering doesn't get mentioned much. I think you should use your own judgement.
- Either will work. You might not want to have the protector firmly attached to the spokes by bolts or other fasteners that alter the stress points of the spoke. But maybe nylon zip ties or something. Or fasten it to the big cog.
- Protector isn't to prevent chain derailment. It's there to protect the spokes in the event the chain does come off the cog while you are sprinting with all your might from a dead stop. Other times you probably aren't putting out enough power to damage the spokes. Unless you foolishly keep pedaling. Again, the DR low limit are what keeps the chain from coming off the big cog and correct adjustment and condition of everything else. IE, the DR hanger.