The DIY will work, but mine kept popping off. Start the search now for an real tester. Start a thread here, and ask locally from other owners, pro shops, etc. I got one off nation luckily, but I was able to use the pro shop's one while I searched. Took about 2 months.
I made one out macro line as posted above. Works great if you have the right macro size. I could see if you don't have a bunch of macro line sitting around that it could be flustrating to find one that fits. Was shocked that is seals so tight without an o-ring. Would have purchased a real one, but could never find it..