here's my take on a diy luggage rack. i asked on another thread about carrying luggage with a passenger. with no affordable options, i took a trip to home depot and $30 later, had a rack that held up to some extreme storms and 1200 miles.
so to start, the simplest design is 2x 30" pipes, 2 elbows and a 8" pipe. the problem with this is that it blocks the brake light. you can relocate the brake light, not care, or build around this. i built around it by moving the luggage part upwards. thus the more complicated design.
its basicallly some black 1/2 inch pipe, t-ends, elbows, and 2x 8"nipples as crossbars to create this. the only problem with the design if that all the pipes are right hand threads (righty tighty, lefty loosey), this prevents you from building a box. don't see why? go to HD, pick up 4 pipes, and 4 elbows. try to make a box and you'll see the issue. you can resolve this by threading one pipe fully, then start unscrewing it as its screwing in on the other end. (i realize this sounds a bit complicated but you'll see what i mean if you have it in front of you). I thought this idea was very sketchy. i didn't want my rack falling apart on my trip. so my way around this issue was to make a crossbar differently. above the lower t-pipe is a 3.5 inch extension nipple. what i did was buy a 3/4 t-pipe with 1/2 reducer. this is a T-pipe where the two of the ends are 3/4 inch and the 3rd leg of the T is only 1/2 inch. i connected 2 of these with a half inch diameter 8" nipple. then this slid over the 3.5 inch extension on the rack. this creates the top crossbar. of course if your store stocks left hand pipes as well, this can all be ignored.
sorry if the explanation is pretty crap. i've been meaning to write this up for a week and been ignoring it. i figure a half ass job is better than none for those who wanted to see it.
i wrapped them with some foam just in case my wife's back was resting against it and to prevent the cords from sliding much. the foam is sold in the same section. it was like a dollar for 6 feet or something.
the black cast iron elbows were all rusted as it rained heavily during the trip.
i'm sure you can design something better but hopefully this will help you with a starting point.
shopping list was as follows (from memory)
2x 18" black half inch pipe
2x 12" black half inch pipe
2x 8" black half inch pipe
2x 3" black half inch pipe
2x 3.5" black half inch pipe (why a 3.5" and a 3"? because 3.5" was on sale. seriously. you could use 4x 3" pipes.)
4x half inch elbows
2x half inch caps
2x half inch T-fitting
2x 3/4 inch to 1/2 inch reducing T-fitting
the long pipes were inserted between the grab handles and the seat. it worked fine for a 2000 750 but ymmv. tied down with paracord and safety wire for good measure.
sorry there isn't better pictures of the setup.