As a miner I went through many different DIY frames designs and everytime I was learning something new about them. Eventually i've taken what's the best from all of them and put it into one. As a result I created frame which i called "Double Decker". In my opinion it's the best in terms of value and capacity. Because GPU's are on two levels instead of one, all standard 0.6m raiser usb cables are long enough to reach the motherboard. Also it's easier to orginise some cooling when using it in 12 GPU's setup .
Frame is made of 20x20mm stripwood and i've used 10mm MDF board as a base for the motherboard and PSU's.
It's capable to fit up to 12 graphics cards in 6+6 setup with 60mm air gap between them(90mm centres)
Or 8 GPU's in 4+4 setup with 120mm air gap (150mm centres)
Don't spend your money on expensive frames, spend it on the hardware. If you are living in the UK, material will cost you something between £10-20 depending on where you getting it from + an hour or two of your time to make it.
Apart from the stripwood you will need 18 4x40mm screws for the frame and 18-22 3x12mm screws for motherboard and graphics cards. Also, I've used 5mm ruber spacers for motherboard but it's optional. Obviously a saw and a screwdriver. On the pictures below you can find all the dimensions and list of material. I'm not a designer so if anything is missing on the drawings or it's not clear just ask in the comments.
If you are not a DIY person and like the frame, you can get it on my auction here.