OK, here's a DIY for your 42" 5-1:
Materials: (stainless steel) 1- 1/4" by 20 1" coupling nut, 2- 1/4" by 20 1 1/4" carriage bolt, 1- 1/4" by 20 1" carriage bolt, 3- 1/4" washers. 1- 10' by 3/4" schedule 40 (the heavy duty) PVC, 1- 3/4" "T" fitting, 3 3/4" "90 degree" elbows, 3- 3/4" caps, 1 tripod to hot shoe adapter, 1 can krylon plastic spray paint- black, cleaner and glue for PVC, to hand clamps large enough to clamp your 5-1 to the PVC and a bit of masking tape
Tools: Electric drill, 1/4" drill bit, triangular file (small enough to go inside a 1/4" hole, pliers, tape measure.
Cut a 20" piece of the tubing and glue a "T" to one end and place ( don't glue it yet, wait until you've made sure that the flash is close enough)a 90 perpendicular to the "T" at the other. Cut 2- 20" pieces and glue them into the "T". Glue on the other 2 90s parallel to the one on the back of the 20" tube so that the open ends are all pointing in the same direction. Cut 2 more 20" pieces and glue a cap on each.
Drill a 1/4 inch hole in the middle of the remaining cap and from the inside run the 1" carriage bolt with washer through the hole so that the threads are on the outside and then put on another washer and screw down one of the 1" coupling nuts so that it's good and tight and then glue it to the remaining piece of PVC. Now place the tripod adapter into the coupling nut and attach the flash and measure from the flashtube down until you reach 20"s and make your cut there.
Do not glue the pieces of PVC with the caps into the elbows, you'll need to be able to take this apart for storage.
Assemble all of the pieces with the flash attached to the end with the elbow not the "T" (you might need to lube the elbows with white petroleum jelly or sand the parts with caps down just a little- you don't want the thing to fall apart but you do want to be able to take it apart. Clamp the 5-1 to the arms of the device and find the balance point. Mark that and drill a 1/4" hole from the top straight down. File the hole square so that the carriage bolt doesn't twist and put the 1 1/4 carriage bolt through the PVC from the top down then put on the last washer and coupling bolt and tighten snugly.
Now you have a holder for your 5-1 that can be mounted onto a tripod or light stand or even hand held. The flash is far enough back that (if set to 28 degrees) it should fill the whole 5-1 to be bounced Straight back onto your subject.
If you have an umbrella mount for your light stand then you can tilt the holder up and down as well as the 1/4" coupling bolt is big enough to be mounted to it as well.
As a bonus, if your 5-1 has a diffuser panel in the middle then you can shoot through the diffuser and get great results up close and since you should have the flash far enough back you won't get any hot spots. This is where you make sure that the flash is close enough to fall within the boundaries of the 5-1 as you don't want raw flash hitting your subject, after you're happy with it remove the flash/5-1 and paint.
Enjoy it,
mike
Oh, by the way; If you already have an umbrella set up and are wanting the flash tube to be centered you can go to the hardware store and get a pair of 3" or 4" angle brackets and a couple of nuts and bolts and a 1/4" by 20 by 1/2"bolt with the coupling nut and washers as needed.
Bolt the coupling bolt to one of the brackets so that it's pointing in the same direction as the bend for a "U" shape and the strap to the downward side then bolt the other angle bracket to the strap and then the strap to the other angle bracket so that the other side of that is pointing away to make a ledge. Mount your flash to the ledge using a tripod adapter and a 1/4" by 20 nut and then drill a hole above the flash head to put the umbrella's shaft through and into the holder. Put the coupling bolt in the top of the umbrella holder and tighten, the umbrella through the bracket and into the holder and you're good to go. Adjust as needed.