You don't need a jig if you make your own side plate and have a half way decent feel for straight and level. If you don't, then drilling the lock nail holes will be the least of your problems.
Once the bolster is in, I drill the rear lock nail hole to use it to remove the lock during the inletting. I drill my pilot holes (tap drill size) from inside the lock mortice to make sure they are where I want them on the lock plate. You need to make sure the front lock nail (if you make it by forging, a lock bolt is made exactly like a nail) will clear the ramrod. This means you need to know where the top of you ramrod hole falls inside the lock mortice. The front lock nail needs to be above this. If the lock nail will fall too close to the edge of the lock plate, You can file away some of the lock nail after it is fitted.
To drill the hole, I camp the stock vertically in the bench vise with the lock mortice facing me. You should have already located and center punched where you want the lock nails to exit the lock mortice. Then it is just a matter to dill the holes with whatever bit you prefer (twist, brad point, small auger (for brace), or gimlet). I believe it is best to do this with a hand drill, brace or gimlet. Assuming the stock is perfectly verical, you want to drill level and perpendicular to the line of the bore. You shouldn't have shaped your lock panels yet, so it doesn't matter if you are a little off. If you are building a kit and must use a pre-made side plate, drill a little better than half way through from one side and then from the other side meeting in the middle. All you need is a square to locate the exit holes.
Once the holes are through, turn the hole thing around and clamp the lock in the mortice. Now take that tap drill bit and mark the hole location in the lock plate. Drill the tap drill holes in the lock plate on the drill press.
Before you put the lock back in the mortice for tapping, drill out the lock nail holes with a clear drill. Put the lock in the mortice and tap the hols through the lock nail holes through the stock. Now, you can screw the side plate to the side of the stock using the rear lock nail leaving the side plate to spin around. When I cut and file my side plates, I leave the front end a little long and wide. I orient the plate the way I like, hopefully it looks like the front of the side plate sufficiently overlaps the hole you already drilled. If, not, well this is where the art comes in. As we were always told in art school, art is all about problem solving. So, get busy being an artist.
Assuming it looks like you have enough metal at the front of your side plate to do what you want, clamp the side plate in place and remove the lock. Now, use a clear drill and drill through the front lock nail hole from the lock side marking the inside of the side plate. Just as with the lock plate, remove the side plate and drill the front lock nail hole on the drill press using a clear drill. This gives you a final opportunity to make sure that hole is where your want it in the side plate BEFORE you drill that hole.
Now, I usually start out with drawings of all my mounts, so I usually finish all but the front 1/3 of the sideplate. If your want to leave yourself the most room for error, just a pick a huge ass piece of scrap metal from which to make your sideplate and only drill the hole for the rear lock nail. Then, you just drill the front nail hole and cutout the side plate around the two holes.
At this point, you should have a partially inlet lock with drilled and tapped lock nail holes AND a sideplate blank with two holes for the lock nails. Just finish the sideplate and screw it to the stock and scribe to inlet it.
Easy peassy, and NO special tools or jigs. It took more time to write this than it takes to actually do what I described.