Unicrowns are hard to mitre by hand it’s true. I usually use paper templates (which are very accurate) but you can’t wrap those around unicrown blades properly because it’s all curved right where you need to put them.
I’ve made two unicrown forks. On the first one I did just mitre it by hand with no template. It didn’t fit up as well as I would have liked (but I welded it out fine and it’s got quite a few miles on it now with no issues). For the second I made a simple mitring tool using the arbor from one of those cheap tube notchers (which I bought but don’t use for anything else). That was fine.
Since then I’ve made segmented forks. The first one was because the guy the bike was for preferred it (it does look more “deep custom”) and after that I also found I liked the look myself. You also get much more choice of blades to use than with unicrown (and I really like curved blades).
Since I already have the mitring tool I made, I still use it: I attach the “crown tubes” to the legs first, weld those all the way out, and then mitre the middle part with the tool.
But if you didn’t have the tool you could use paper templates on a segmented fork easily, because the “crown tubes” are just straight tubes. Do all four mitres with paper templates before tacking anything. This is how I do segmented seat stays which I’ve done on a couple of builds, and they’re just like forks, only at the back (and with much thinner “crown tubes”). In many ways it’s easier because you can make sure the two little tubes are exactly the same length more easily if you can hold them in your hands before they’re attached to anything.
As for fixturing forks, I found the tool I describe below to be the key. With that you can get away with a less than perfect fixture. You can anyway (it just takes more patience).