For my amp rack, I used 3 pieces of 36" long, 1/2" thick, threaded dowel rod from Home Depot and 2 24" long, 3" wide, and 1/8" thick pieces of welding steel to create the brackets. Home Depot also has a 2" wide nut that u can use to connect 2 pieces of the dowel rod, as well as standard 1/2" nuts.
I put 1 of the dowel rods in a vise, between 2 boards so it wouldn't damage the threads. I put a 90* bend in the dowel rod so I had a 24" section, a bend, and a 12" section (the exact measurements will depend on the size of your amps, so u may have to tweak these #s). I did the same thing w/ another of the rods. The 3rd rod remains straight.
Then I fabricated the brackets. I cut 4 6" long pieces of the welding steel, then bent them in 1/2 w/ a 17mm socket inside them. This created a nice round bend at the fold. I drilled a 5/8" hole thru the round part, and another hole thru the flat part. That way, I could put a standard 1/2" nut inside the bracket and stick the end of the dowel rod thru the hole and attach the nut.
Time for a trial fit... bolt the brackets and bent dowel rods to the rearmost (ie, closest to the rear of the car) strut tower nut. Take a measurement and cut the 24" sections of the dowel rods so u can bolt them together in the middle w/ the 2" connecting nut.
The other dowel rod uses the same type of brackets, but I bent and trimmed them for a better fit. It bolts to the centermost strut tower nuts. Once u have all the dowel rods and brackets bolted in, measure the distance between the rods and compare this w/ the length of your amps. Don't worry if it's off by a few inches -- we're gonna build another set of brackets to mount the amps to the dowel rods.
Creating these brackets is similar to the brackets that bolt to the strut towers... use the same bending process to fold them in 1/2, except that u should use a smaller socket -- we want the rounded part to slide over the 1/2" dowel rod, so the socket should have about a 1/2" outer diameter. We're also only gonna drill 1 hole in this bracket -- this hole will hold the mounting screw for the amp. So make sure u drill in the right place and make a smaller hole. When these brackets are done, slide them over the dowel rods to the correct location.
I also wrapped the brackets in black electrical tape, and covered the dowel rods w/ red wire loom. This complemented my red/black w/ white accents color scheme.