You don't really want to make a backplate out of steel. It tends to gall the spindle threads. Blank cast plates are affordable if you want to finish one yourself, but it doesn't sound like that's what you want. Just buy a 6" threaded backplate and fit it to the chuck of your choice. You wanted to know if the cheap imports are any good. That depends on your expectations. They will hold a workpiece. They'll have some run out. You have a 4 jaw so that shouldn't be a problem. The two piece jaws are definitely the way to go.