Rolling magnet was by far one of the best purchases I made! And yes harbor freight has them for cheap. I got mine when it was on sale. Think I paid $10 for it. I roll it over my driveway frequently as that's where I do most of my construction work. Saved myself having to get new tires a few times.
What about a sinking deicer? I use a bucket deicer in one of those waterers with a float in the bottom. Hate that waterer but needed another one this winter. Bucket deicer stays at the bottom so the float can stay free. Only uses 80 watts too. The other heated waterers use 110 I think.
https://www.familyfarmandhome.com/bucket-de-icer-570530.html
This is the one I use. I think its good up to 6 gallons. I use it in 3-4 gallons of water. Of course there are bigger ones too.
I had (recently failed) a sinking deicer for the red 5(ish) gallon water dish. This stock tank says to only use a floating deicer with a guard on it. Hubs would freak if I melted a hole in it.
I wish I had read that part before choosing this tank.