A few weeks ago I decided to create a cheap raised bed in the poly-tunnel, the benefits being that you need to water less often than for peat bags or pots, ideal if you are going away for a few days in the middle of summer. The other benefit is that you can give the growing medium new life every year with new compost, which you can not do so easily in the soil, plus you can totally change it every 3 or 4 years if it get infected with with diseases, or to totally regenerate the growing medium.
I used the same breathable membrane as for lining the floor. Then knocked in wooden posts on either side for support. I ran a wire across the top of the post to lay the membrane over, having it wide enough to fold over and lay on the base.
I then secured the membrane to the wire using a staple gun and bits of thin plastic folder over, I did this before filling the beds. So the water would not drain fully or to quickly I added a plastic strip down the bed, making sure it went a few inches up the sides, but added some small slits for drainage.
A base of small stones were added, then some well rotted compost and leaf compost and horse manure, as we have lots!. I then added cheap bag fulls of supermarket compost, but to this mixed in some slow release fertiliser. The total cost was about 20€ max.
I planted a few chili plants to start with, which are doing very well. I will feed once a week at least with a high nitrogen fertiliser, as this washes out of the soil easier than other chemicals. Later I will use a standard tomato feed. I have used growing bags for the rest of the poly-tunnel, but will eventually have raised beds for all my plants.