I use BBT style cases built out of two 8ft pieces of 1/8" x 3/4" angled aluminum, one 4ft pieces of 1/8" x 3/4" angled aluminum, one 8 ft 1" x 3" piece of pine, one 4' 1" x 2" pine and #8 1/2" self-tapping screws I get at Lowe's. The case will easily fit 7-8 RX 4XX/5XX if you put them about 1.5" inch apart. The cost of the aluminum angles and wood is ~$45. If you build it out of all wood, the cost would be less than $10. It takes about 1-2 hours to build once you get the hang of it.
https://www.youtube.com/watch?v=D5xNrlxsCVshttps://www.youtube.com/watch?v=MA_sH4q2toU&feature=youtu.be&t=2837-8 GPU Frame (Dimension 24" W x 15 3/4"" D x 10 3/4" H):
Build Cuts:
For the Angled Aluminum:
-First 8ft (96") Angled Aluminum; 5.75 inch of waste
4x 15 3/4" cuts (58" use) (Case Depth)
3x 10 3/4" cuts (32.25" use) (Case Height)
-Second 8ft (96") Angled Aluminum; no waste
4x 24" cuts (96" use) (Case Width)
-4ft (48") Angled Aluminum; 37.25" left over
1x 10 3/4" (10 3/4" use) (Case Height)
And for the wood:
-8ft 1x3 Pine (96"); 9 inch of waste:
6 cuts of 14 1/2" (87" use) (Case Bottom)
-4 ft 1x2 Pine (24" left over):
1 cut of 24" (24" use) (Card Bracket Support)
I also built a 14-15 GPU double tier case for a new build I'm doing with the ASRock H110 BTC Pro mining motherboard. About $60 in angled aluminum and wood.
14 GPU Double Tier Frame (Dimension 28" W x 17" D x 17" H):
Build Cuts:
For the Angled Aluminum:
-First 8ft (96") Angled Aluminum; no waste
4 x 17" cuts (68" use) (Case Height)
1 x 28" cut (28" use) (Case Width)
-Second 8ft (96") Angled Aluminum; no waste
4 x 17" cuts (68" use) (Case Depth)
1 x 28" cut (28" use) (Case Width)
-Third 8ft (96") Angled Aluminum (96"); 12" left over
3 x 28" cut (84" use) (Case Width)
And for the wood:
-1" x 3" 8ft (96") Pine; 11" left over:
5 cuts of 17" (85" use) (Case Bottom)
-1" x 3" 4ft (48") Pine; 14" left over
2 cuts of 17" (34" use) (Case Bottom)
-1" x 2" 4ft (48") Pine; 20" left over:
1 cut of 28" (28" use) (Card Bracket Support)
For cooling I use 20" box fans in the front to keep good air flow through the rigs.