You will need more than a jar lid to stop its power from gabbing things.. Keep it away from your truck when walking by as it will latch on and tear it up. Get a good rope also. I have several and have two mounted to the bottom of wood handles from a wheel barrow. I walk in the creeks and probe with them. If it gets stuck all I have to do is turn it sideways to release it. Also serves as a waking stick in the creeks when slippery. I have picked up cut pieces of rail road track. Found some really cool old signs, rusted out guns and tools and fishing gear. I will sometimes clear an area of iron with it then cherry pick targets with my metal detector. I also get old rail road steel with high carbon for my forge. Found a safe face down and have yet to get it up as its in deep water.
Just be careful around your fingers and wear gloves to clean the magnet off.
If you are throwing it with a rope wait till summer in case you get hung up and then you can swim out and get it. Figure if the magnet says 1000 lbs hat is 500 lbs per side.
Have fun its a great way to spend a day. Glad to see people using this area of the forum. We were the first on the net to have a magnet fishing area.