This is the "for cheap" way so highly honored here at Mouldy Acres:

If you drill a vertical hole through the 2X6 boards you can use a small rebar through the holes and down into the ground to tame any bowing. However, since the bottom board is constrained from bowing by the soil it sets slightly down into, you only need to tie the upper board to the lower, and you can use 8 or 10 inch spikes too..

This plastic stuff also takes screws well, and any cheap framing bracket in the inside corners attached with 1" or so drywall screws should be pretty much unnoticeable.

hmmmm, dont seem to have 2x6 around here, only 1x6's!

thanks.

Would 1x6 be to flimsy?

Not sure about composite decking. Is it treated wood? You don't want chemicals leaching into your food supply.

Why not used Cedar??

1 x whatevers are fine, but do you really need to make them 20 feet long? I'd use cedar and use 12" wide boards rather than two 6" wide boards. Just seems like it would be cheaper and a lot less hassle. Cedar will last years and it will not put chemicals into you food.

I'd recommend splitting your 20 ft long beds into 2 ten foot long beds, especially if you were to use cedar.

I made a 12 x 3 bed out of 1x12x12 cedar last year. I buried the lower 1/2" to keep the bottom from bowing and then just ran a 1x1 cedar post between both sides right at the midpoint. This sucks both sides in a the top and aesthetically, creates separation in the box.

If you are dead set on using composite decking material, then just run a few 1x1 railings posts between the sides.

I know, picture quality not so good, but it was through a screen with my phone, so didn't expect much. Anyway, you get the idea.

And yes, it is snowing in Minnesota today!!