Wow, those are some interesting battery terminals, especially since the positive looks like its black as well. If you want that splice to be permanent, have someone solder it and use shrink wrap. Hopefully someone didn't hook them up backwards in the past. I don't think you are looking in the right place. That engine grounding strap "hanging on by a thread" looks reasonably secure, and also consider there is another one right from the negative battery cable. The one in the picture is more or less an extra just in case. Plus it looks like it has minimal corrosion, and your other connections likely look good as well.
If you really want to continue down this path, get a proper positive cable. Also clean the top of your battery, dirt actually can drain your battery over time. Throw the donut under the negative battery terminal out, they don't do a thing, and put dielectric grease on the terminals to prevent corrosion.