I would build a wooden frame of 2x4's, of some durable wood or preserved in some way preferably but sometimes of junk wood i know will only last a season or two. Drill holes in the top to take pvc hoops. More often, i have simply jammed the pvc pipe into the ground. We just did something of this sort a couple of days ago here. I use regular clear 6 mil poly for a covering. If this is protected from summer sun by using same apparatus for shaded beds, it will last a couple or 3 years or so. Get rid of it when it starts to being brittle or wish you had. You can use 4 mil for a year or so. I just pin the sides with rocks, which are all too available here, or dirt or old lumber depending on circumstance. The one we just made uses 10 ft wide plastic, and has a spine of 1x2 which the hoops are tied to, and then the end hoops are pulled in opposite directions with a small rope and stake. The plastic is long enough to pull down the the ground, with small hole made for the rope ( reinforcing this with duct tape a good idea)

Crude but effective, as i often find mycellf saying.

I would first do double digging then take and surround the area with concrete blocks.

I would get 1/2 inch PVC pipe and loop it in half circles approximate 12-15 inches apart anchoring it inside the concrete blocks

I would then fill the concrete blocks with sand

I would do a homemade mix for the semi rasised bed you have created of:
(lets say 8x4 1 ft deep)

nine 5 gal buckets of leaf mold/peat moss
six 5 gal buckets of vermiculite/perlite
four and one half 5 gal buckets of builders sand
8 cups of wood ashes/charcoal
4 cups lime
4 cups organic fertilizer
four and one half 5 gal buckets of compost

I would than go to the local hardware store and get 6 mil plastic that can be wrap over the PVC pipe and ties at both ends.

Rather thatn stapling I would take the PVC pipe and create mini clamps by cutting it in 3 inch pieces and cutting an opening in each piece so it can snap over the plastic without damaging it. (Several Gardening catologs have fancy made PVC clamps that can be purchased if you want it to look better)

You have alot of good advice here. A couple of really simple ideas, on a double dug raised bed, doesn't need any container around it, is a clear plastic on a wooden frame of any nature, lat material is fine, that stands over the bed. Another way is by actually digging in about three to four feet a central framed area, then extended plastic with wooden lat frame around this area, usually A-frame style. I covers the beds and you can step into it without disturbing it. In the summer take the cold frame off.