To build and fix a custom door on your car that fits safety requirements and will fit your preference is not difficult if you follow the steps below.
First, make a template for the door opening from cardboard, which may be subjected to adjustments later on. Next, cut a two inch strip form 18-gauge cold rolled steel and bend it into 90 degrees, with dimensions of 1x1 inch. Fabricate it with the template as a guide and tweak it into perfection against the real opening. It will form the inner door perimeter. Then, bend another strip into 90 degrees, with dimensions of 1x2 inch, forming the outer perimeter. In the inner structure, punch holes along the horizontal flange for welding later on. Slip the outer flange over the inner one, allowing adjustment for the width of the door. Check and tweak the pieces until they fit the template exactly.
Now, clamp both perimeters together and weld the bottom of the door too, where the pre-punched holes were. Then put the frame back into your vehicle to check its fit as well as to trace the proper hinge location and mark the attaching points. Original woody hinges are recommended if they are the correct shape and small enough. They will be undetected after being covered by the installed wood trim.
Drill a radius at the corners using a small hole saw and remove the excess metal using a cut off wheel. Now, the cut-outs should be completed. For the backing up of the threaded fastener plate, you can fabricate a "stair step" holder with a front cover which features access holes for the installation, removal or loosening of the bolts when adjusting the door into perfect fit.
Along with the fastener plate as well as the holder, fit the frame with the hinges and weld them in place after precise placement. You should have a functioning swinging door. Lay out and mark 20 gauge steel for your lower beam support, then bead roll a number of strengthening grooves so that the door assembly can be stiffened. Using a 36-inch roller, roll the ½ beads out on both the inner and outer panels, following the markings you just made.
With the help of a Tenn Smith brake, bend the panels so that one can be fitted inside the other, the fit provided by the 90° lips. A rectangular beam will be formed by the two pieces, stiffening the door and allowing fastening spots for a power window system.
Spot weld the stiffening tube together, and cut the back half on each end out and install them to the door, so that stitch weld can be done. Then, cut strips from 20 gauge and lay them out for two qty pieces, and bend them at 90°. Weld the 2 channels together so that the window channel rear portion can be formed, which is where the glass's rear edge will slide vertically. To achieve desired width, use ½ inch square tubing as form, and clamp the two sections in place for welding. After welding, leave it to cool.
After fitting the channel to the rear door edge at the driver's side, you can see that ample clearance is allowed for the door catch which will be mounted later on. Then, install the channel and weld it to the passenger side door. Build channels measuring 2x1 inches, with a small lip bent on the 2 inch side. Fix the interior window sill to the frame and weld it in place. Now, trial fit and cut the window fuzzys to length for the mock up later on. Build a channel as well as pie cut a section. For matching of the roof line, shrink the upper section. Install a 3/16ths wood for temporary glass to check fit, if clearance is right, tack weld the channels properly. Spot weld the inner and outer frames together with the top window channel. As you can see this is not an easy task to build a one of a kind door but at least you know the correct techniques to expect if you have to choose a street rod shop to build it for you.
Finally select small bear claw latches including door panel corners that are pre-notched for the latch. Cut a notch at the edge of the rear door so that the unit could be fitted in. Fabricate a cap to finish the cut out and weld the cap and the latch plate in place. Ground it smooth and you are done.