OK, just something I was working on, I hope you find this helpful

If you would like any other info, let me know, I'll be glad to help
I hope this will help you build one of your own similar to the actual movie version, or a simplified version that will work just as well.
Lets discuss the harness. The harness is thought to be a SCBA harness that would have been made some time before 1979. The harness itself seems to be made out of polished stainless steel that has been spot welded in certain points and altered from its original form in order to accommodate the Jet Pack. What seems to be altered on the pack is the absence of a loop located on the bottom of the pack that would have held the valve portion of an air tank, the straps that would have held a tank, and the belts that were part of the original harness system. A ¾” wide strip of metal has been added to the body of the harness just along the top of the lower bracket that is about 8-10” wide that is held onto the tubular portion of the harness body with 2 slotted head screws, one on each side of the harness body. This strip of metal was added to support the lower portion of the Jet Pack. The Jet Pack had 2 slotted hooks located at the bottom of the back plate that hooked onto this strip and rest on top of it.
The harness itself is made out of a piece of ½” tubular steel bent into a “U” shape that is approximately 4” wide and 14-16” long. The very top of the “U” shaped body seems to be bent at a slight angle to contour to the back and shoulders. Located on the bottom of the harness is a bracket that is about 2” tall by more then 4” wide and made out of 2 pieces of stainless steel that are coiled around the tubular steel frame. The 2 pieces of steel that wrap around the tubular frame are made of 2 distinct shapes that are spot welded to each other. The first piece runs the width of the harness frame, and wraps around the tubular frame in a “P” like shape. The second half of the bracket fits right under the first piece and is “V” shaped on either side, in that piece of the bracket the diamond shaped rings are inserted. This bracket is mounted to the tubular steel frame, and also supports 2 large steel diamond shape rings that the harness’ belts are attached to. The diamond shaped rings have an inner dimension of 2” square from inside corner to inside corner. The rings attach to the bracket on either side of the tubular frame.
Along the body of the frame from the bottom to the top, there are at least 2 more supports that run the width of the tubular steel frame. The support at the top seems to be at least 1” wide, while the support closer to the bottom bracket seems to be at least 2” wide.
The straps attach to the diamond shaped rings on two of the four sides of each ring. The belt strap is attached on the side parallel to the frame. The shoulder strap is attached on the side perpendicular to the frame. The shoulder straps attach to the ring, wrap around your waist, and then reattach to the top of the tubular frame. Two additional pieces of strapping are attached to the top of each shoulder strap and extend out the back plate. At the end of these pieces are rectangular rings that help support the top of the jet pack. The top of the jet pack hooks onto the rings on these strap. As shown in some of the reference pics, the black belt for the waist strap can be seen, along with the straps attached to the rings for the shoulders, but the strapping that supports the Jet pack is a different color and material. These strap extensions can be seen on one of the chronicle pics. The shoulder straps most likely were held together with a metal slotted plate assembly that was permanently attached to one half of the strap and laced thought the slots with the other end and could be pulled down to tighten the straps. The top portion of the shoulder straps were looped around the top part of the “U” shaped frame and most likely riveted together.
The harness worked buy hanging the top two hooks located on the Jet pack onto the loops from the straps attached to the harness shoulder straps. The bottom two hooks located on the back of the jet pack would then rest on the bar that is attached to the lower bracket on the harness.
The whole series of pictures can be viewed here
http://ImageEvent.com/fettcicle/jet packharnesspics