Here's a quick way on a no-zig-zag machine, and I think it's stronger than a bar tack anyway: Just go straight front to back a few times (forward, reverse, forward, reverse) but don't go over the same stitches again and again (weakens the fabric, as was said earlier). So you're making a W if you do 4 lines. If you want to strengthen it more, then do a box around the W afterwards. You can do the whole thing without having to cut the thread, and you only need to turn the fabric to do the box.