Weld Nuts - various threads
Available in 1/4-20, 5/16-18, 3/8-16, and 1/2-13 threads. Priced individually - buy what you need.
These threaded plates can be welded at each end, minimizing the risk of distorting the thread. The flat part on either side of the threads is 1/8″ thick, and the threaded portion is 1/4″ thick.