I'd recommend to use the provided seat belts, as we do.
If your makeshift latch system fails in an accident and your child gets seriously hurt or killed, you will 1) feel like crap, 2) most likely be charged with negligence or manslaughter, and 3) make national headline news, complete with bad police photos of your jury-rigged LATCH system. They will most assuredly also dig up your post here (and any subsequent posts) to be used as evidence at trial, or simply to be quoted (and misquoted) in the news.
Child restraint systems are designed and manufactured and put through batteries of tests by degreed professional mechanical engineers, and the auto makers shoulder the liability if something goes wrong with those systems in an accident.
It's just not worth it for a little convenience (and really, having used both, LATCH is only slightly more convenient than seat belts).
Tent camping for now. Dreaming up a new 4x4 SMB. Gas or Diesel?