This is a quick frame I scrapped together to hold some extra graphics cards while I test them for mining Ethereum though it should work longer term setups as well. I just go over the key decision steps about clearances and spacing since the design is simple enough to assemble and modify for different setups.

Some background you are free to skip if you don't care/already know:

CryptoCurrency mining has had a big resurgence in popularity, especially among people that have never done it before. Ethereum and ZCash are two coins that have become profitable lately driving the national stock shortages of AMD RX 4 and 5 series upper tier graphics cards (the best price/performance ratio for mining said coins). I have some rx 570s, but GTX 1060s are giving me about 24MH/s and running at a breezy 58C for $20 less per card so I may change course in the future

Image Credit: Bitnewsbot.com "What is the Ethereum difficulty bomb?"

Step 1: Planning Clearances

Before you begin, decide how many cards you are going to put into the frame, and based on how hot they tend to run you can determine the necessary spacing (I am currently using MSI and ASUS GTX 1060s and chose 4" spacing between cards).

An important distinction is whether you plan to put your motherboard beneath the rack or not. Putting it beneath the rack requires more vertical clearance, so just raise up the legs an extra four inches to get the necessary clearance if you choose to go that route.

If you don't want to use a wooden slat spacer under the cards you can replace the 4.5" dropdown measurement with 4.4" intead

Step 2: Mounting Bar

We start out with a piece of 3/4" Aluminum L channel. Since I am making a 4 card setup I need 3x4" (spaces between cards) + 2x3" (spaces on the ends) = an 18" piece.

Computer cases and standoffs normally use #6-32 screws, so I followed this convention and used a 0.1470" drill bit followed by a 6-32 tap. I placed a card on the L side to measure off my holes, but make sure your holes are 0.16" away from the edge to ensure the graphics cards' mounting plate fits flush

Step 3: Rear Feet

For the rear feet I used some scrap that was laying around, the length is the only driving dimension as long as you make it flush to your side panels and follow clearances.

I used a miter saw to cut my 2" lengths for the rear feet and then sanded the rough edges with 120 and grit sandpaper

Step 4: Rest of the Wood

Continue following your plan for the rest of the wood and screw it all together with wood screws (make sure to predrill to avoid splintering).

I chose to assemble everything except the rear feet first. Clamp your pieces flush to keep the piece even.

Step 5: Attaching the Mounting Bar

Drill holes in the ends of the mounting bar and attach it to the front posts with a last set of woodscrews.

I sanded the top face of the aluminum with 400 grit sandpaper to make it have a nicer surface, this is totally unnecessary but it looks so much better :)

Step 6: Card Spacers

Depending on what PCIe risers you have there can be up to a millimeter of variation in height, so I used a 2mm balsa wood spacer to make up the last bit of height to support my risers.

Step 7: Add Your Cards and Mine

That's it! Just put your cards and spacers to get started. If you don't have passive cooling for the part of the room you will be mining in I strongly recommend adding mounts for some PC fans on the frame (I'd be happy to do a guide for that too)

Happy mining!