I've seen some great chassis and setups out there and found that it's extremely easy to build your own open air chassis for next to nothing.
First, for your base, go out, get a scrap or square of plywood for a few bucks at your local hardware store. Mark the plywood with your motherboard hole layouts and then drill out the holes with a 1/8" drill bit. The small metal motherboard standoffs will screw right into those and you can mount the motherboard to that.
Your power supply can rest right behind the board on the plywood. As for the GPU's, it depends on whether you want to leave them on the motherboard or pull them off the board with extenders.
If you want to keep the cards on the motherboard, you should be good to go. If you're worried about the cards seeming to be loose because they are being held up only by the slot, pick up an
aluminum bar from the hardware store (you only need about 6 inches, so see if they have a scrap.) Cut the bar to fit the width of your cards by lining it up to the top of the brackets and then use some self tapping screws (about $5) and attach the top of the card brackets to the bar and it'll hold them extremely securely.
You can also pull the cards off the motherboard with extenders. This is my preferred method just for better cooling and airflow. I bought these
1X extenders on eBay and with a pair of snips you can trip the end off of them in about 5 minutes. They're cheap (~$3.50 each) and ship from China in about a week. Then all you need is to find a
simple wire cabinet shelf to support the cards. I found them at Walmart for around $7 USD each. It's sturdy enough to hold up the cards and the legs should be wide enough to straddle the motherboard. You can easily space the cards out to give them some good airflow and the extenders will feed right through the wires. To hold the cards sturdily, just pick up a small spool of bendable metal wire. You can wrap it around the rack and then loop it around the top of each cards metal screw bracket in turn and then back down and around the other side of the rack.
Here's part of my setup with 4 rigs. 2 have the cards off the boards with extenders, the other two have the cards on the boards using the aluminum bar support.