You need a longish bolt, a nut and a washer of the correct size to bodge this scucessfully.
Put the nut on the bolt, then the washer and then the rivnut. Put the rivnut into your now drilled hole. Hold the top of the bolt with a spanner, pushing the rivnut into the hole. with another spanner tighten the nut down onto the washer/rivnut. As you tighten the nut down onto it, the rivnut will compress into the back of the hole.
Much cheaper than the tool I leant to a "friend" that never got returned!