Last year, I tried to fabricate my own abbreviated rear fender. I've also got a Topeak rear rack, which catches some of the spray. So I took a small piece of Kydex (plastic sheeting that is about 1/16" thick and can be heated up and molded) and I cut it to be about 2/25" wide and a little more than a foot long. It ran just from the chainstay bridge up to the rack, attaching at the same place where the seatpost mounts meet the rack. It wasn't beautiful, but it did catch some of the muck.

I ultimately ditched it, though. As has been pointed out, you'll still need a fender up front. So you'll probably end up with a full set. And the DIY solution isn't any less difficult to remove if you find yourself having to take the rear rack off. So why have a 3/4 length rear fender? It isn't saving any weight. It isn't any easier to install and remove. It isn't any cheaper. This was one of those projects that just didn't make any sense. I'm all for rolling your own. But sometimes it's just smarter to buy the pre-fab part that is already made to do the job. That's my two cents.