Matt, I can't help you in regards to a jig, but don't understand their popularity. I've built many turnouts, and I build them in place. I will write the basics, I suggest the article by Tony Koester for fleshing out the details. It originally appeared in a Model Railroader issue, was reprinted in one of Kalmbachs books, you will need to do a search for it.
The basics: Draw your centerlines for the track. This must be done carefully as you will glue ties down on them, and you need nicely flowing lines. I often use a piece or two of Atlas flex track, tacking it down as required and drawing lines along the ties on both sides. Do this for both routes thru a turnout. I find this particularly essential for curved turnouts where I want both routes to flow smoothly. I should mention here that the curved turnouts I've built do not follow prototype practise (I believe) in that I maintain a curve thru the entire turnout, including the frog. Once you have your lines drawn, glue down ties. For the turnout ties, I built jigs consisting of stripwood (use a width suitable for your desired tie spacing) glued to a 1x3, butted up against a stop. The stripwood should be thinner than the ties you use, to make it easier to remove the tie strip. Refer to a commercial turnout, or buy a BK kit, to see how the ties increase in length as you move along the turnout. I cut the ties in increasing increments of 1 foot. When the ties are in the jig, use a strip of masking tape to hold them in place till laid on the roadbed.
OK, so the ties are glued down. You've located where the throwbar will be, and used headblock ties there. One of the nice things about handlaying is that you can use full lengths of rail thru the turnouts, even one piece of rail for more than one turnout in some cases. Lay one of the stock rails in place, mark it where the points will nest into it. This is where Tony's article will be helpful to you, good photos and measurements are provided. I file the base of the stockrail where the points rest, about one and a half ties back from the throwbar. Spike both stock rails in place, using the ties to line them up. Next you will form the frog to fit, no need to worry about what # it is, or need to use jigs. (The # of the finsihed turnout is dependent on how you laid the ties, I forgot to mention that the tie jig I mentioned used dimensions from commercial turnouts) So if a particular turnout is really a 5 1/2 or 6 1/2 rather than a 6, I don't care. You need at least three of the three point track gauges. Lay in a rail gauged to one of the stock rails, after first filing one end for the frog point. Hard to descibe, easier to view a photo in Tony's article. (You really should find the article!) But basically you file the railhead and base off on the side that meets the other frog rail, and just the railhead off the other side. Hold that rail in gauge with a couple gauges, file another rail for the other half of the frog in the same manner, and place it in gauge to the other stock rail. Slide them till you find the point where they rest against each other and are in perfect gauge to both stock rails. Spike. The next step is the point/closure rail, all one piece. A comment here, code 100 rail is a bit heavy for this type construction, I use 83 and 70. The combination point/closure rail has to bend when the turnout is thrown, hence the smaller the rail the easier it is to bend. Again, reference the article, or just practice a bit. You need to bend the closure rail just prior to the frog, so that it runs parallel to the frog as a guard rail. At the frog end of this rail, make a slight bend and file slightly, this will help prevent snagging track cleaning pads and such. Oh, another thing to do before installing the point/closure rail is to file the bottom of the point section (after filing the points themselves) so that there are no rough edges which will prevent smooth movement of the rails across the ties. filing the points is rather obvious, the side against the stock rail need both base and railhead removed, the other side just the railhead. Also slightly round the top of the point itself, again to prevent snagging. Take the time to get this rail correct! Spike in place, do not spike too close to the throwbar of course. I drill a hole in front of the frog point, and install a wire, bent so it will stay in place, hanging down thru the hole. Using a solder gun, I fill the space between the frog and its guardrails and closure rails with solder, which also coats the wire. This will be your power feed for the frog. I use a short piece of hacksaw blade to clear the solder to an appropriate depth. A bit late, I forgot to mention that prior to laying any piece of rail, I use a dremel drill press to drill a hole on the bottom of each rail, and solder in a feeder. Easier and neater than doing so later.
The throwbar: I use copper clad pcb, cut into ties. Drill a hole for your throw mechanism of choice. Use a file to remove copper at some point between the point rails. I use a strip of aluminum foil placed between the bottom of the stock rail and throwbar to prevent soldering the point to the stock rail while soldering the point to the throwbar. I spike the point against the stock rail, with the foil between, and solder the throwbar, repeating for the other side.
Have fun!
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