If you have done them before you could probably do them without the jig. Even the jig isn't perfect. At least the drill press jigs.
The holes on the sides are easy enough, just mark and drill.
But the hole for the trigger sticking out the bottom, that is probably the toughest part.
first few I did I tried to be safe and did not get the pocket wide or deep enough for the trigger to function properly. After you learn and realize how little pressure there is inside that milled pocket you relax and open it up a bit so the parts just drop in.
Yes all the specs are online, if you can trace them out on your lower it could work. I use a drill press and it is a little old and sloppy so for me the jig is more of a safety to let me know when my press is drifting farther than I like.