When in doubt go to Grainger. They got everything, but you will pay more than Harbor freight.

 

Here you go. Take a piece of .120 aluminum or steel and drill a hole just large enough for the bolt to fit through. Take a bolt that is long enough to go through the nutsert, .120 metal sheet a washer and a nut. Install in this order onto the bolt, nut, washer, metal sheet and then nutsert. Hold the .120 metal sheet flat against the tub and hold the bolt with corresponding wrench (don't let it turn). Turn the nut down into the washer, the friction being applied to the .120 metal sheet will stop the nutsert from spinning and the bolt will pull the nutsert up and swedge it into position.

Hope this makes sense and helps you out. I think TeraFlex has a video on youtube with a girl doing this install and using a similar homemade tool. Here is the link. Look at 1:16 into the video. Their tool is way overkill but will give you an idea. My tool is much smaller and easier to put together.

http://www.youtube.com/watch?v=VYhdPapHyxQ

 

I ordered this to install my PSC tube fenders... It was tough to get the rivet started but worked great. The larger arm types work better for the 5/16 & 3/8 threads in my opinion. But for the money I liked this one and I liked that it came with some nuts so I had something I could use in the future.

JEGS Performance Products 80452 JEGS Rivet Nut and Tool Kit