The hader kit from preat works great for hader bars. If the doctor has a scanner use that. You can print out a digital model, trim the model so just the bar remains. Plug that into your alginate impression like an implant analog. Process directly on that. You can also fill the space with light cure. Before you do put housings and analogs in and fill it up
If you dont have a intra oral scanner, block out bar. I use soft red rope wax. Then place new housings and take an alginate impression and pour with implant analogs. If you are not sure or confident about the transfer of fhe housings, make an extra baseplate with hole cut outs around housings, so they stick through top of baseplate. Then you can do a pickup from the top and dont have to worry too much about a lock in.
If they can get you a decent impression it really isn't too bad