so, my bed I made was almost the exact same size.. but I had only one.
I also ordered 9 cubic yards for the bed but ended up only using half of it. I put a rock layer on the bottom for drainage. used a bunch of the sod I pulled up to fill in about a foot of the bottom layer and then used the "garden mix" I had delivered. The garden mix that was delivered was terrible and I had to end up pulling most out and rehabbing it.. but that is another story.
Anyhow, my peppers do fantastic in it.
May I suggest doing something before adding soil.... I put a PVC framework inside of mine before adding the soil and I am so thankful that I did..
The photo below is the framework as it was being assembled. on the top of the uprights I added a slip/threaded fitting and then a threaded cap. I can take the cap off and screw in a tall upright (I use the uprights to add netting around the whole planter, or just take the cap off and slide in a smaller more flexible pipe that can become a hoop system (add plastic over it for a greenhouse). The reason it is interconnected on the bottom is for stability and so I cannot accidently pull it all up during assembly and re-assembly.
Like my huge dirt pile next to it.. yeah that is 9 cubic yards an the truck driver dumped it out in the alley where all my neighbors drive... thanks asshat. I had to spend a morning shoveling it out of the way.
The photo below is the planter the first year with the tomato transplants. You can see the uprights that I screwed in to every other location. Kept that caps on the ones I didn't use so they would not get water or dirt inside (though it wouldn't hurt if they did). If you look close you can see the netting. Oh! and I also added a drip system that is showing in the photo. It can partially be removed in the winter.
Another view from further away.
And with peppers last year (early-ish in season)
