Did that a few years ago. It is a lot of work to be honest with you. Here is how I went about it....
1. I mocked the whole thing up using stiff cardboard. I should have used Kydex, but I really did not know it existed then. Once I had it the way I wanted it, I flattened out the cardboard and started cutting and drilling away. BIG MISTAKE, only cut, do not drill....Restart.
2. I made the lower in two parts: Trigger housing, and magwell, and used a steel bar to mate them, and house the BHO.
3. I used standard pins and cut a slot into each end to accept a c-clip, since the pins are longer than needed (because the trigger housing is only .064 thick) it worked out really well.
4. I made a Frankenstein looking buffer tube / rear trunnion / ugly spud that was spot welded to the trigger housing. This is what the rear pivot pin goes through. Now, think about those stupid little pins and springs that keep the pivot pins from falling out...Yup, the rear trunnion has those, along with the selector - keeper. This rear trunnion was the hardest bit of the whole project. It extended into the trigger housing all the way to the trigger group. It was good though because when I spot welded it into the trigger housing, I can guarantee that it was going NOWHERE....
5. The BHO / mating bar was spot welded to the front of the trigger housing, and then the mag well was spot welded to it as well. The mag well overlapped the trigger housing, and was cut out to allow me to mount the BHO pivot. In hindsight, I would simply have added a tab to the front of the trigger housing that would be bent out as the rear of the BHO pivot, and did the same to the mag well to form the front of the BHO pivot. It would have been simpler...
6. Now for the fun parts.....The front pivot pin went through another block that was spot welded to the front of the mag well, and the stupid little pin that holds it in was drilled from the top....Looked really funny, but it worked. A pressed steel flat was made for the BHO pivot. I used a modified L1A1 pistol grip and a loose nut to attach it. I also used an L1A1 trigger guard as an afterthought.
That was phase 1 of my infamous "Freedom Rifle" concept. It was heavy, and it was ugly. I truly shed tears when I destroyed it.
The good news is that it worked, flawlessly, which was a miracle in itself.
I revisited the concept again a couple of years later, this time I was out to make my own version of the AR-180 that would use as many AR-15 parts as possible. That worked too.
Both projects were fun. Both were successful. I never did anything with either idea because it costs more in tools and time than just buying an 80% completed lower.
However, for someone like yourself who already has the ability to press a receiver from a flat, has access to a spot welder, has the ability to install rivets, this is a really fun and rewarding project. All of the measurements that you need are available if you have an AR lower.
Let me know how this goes...
Regards,