Recommendation For A Coldframe

I live in a flat and do not have my own garden, so a greenhouse is out of the question. I am looking to buy a coldframe so I can harden off my Delphs, Lupins, Dahlias etc.
Any recommendations what to buy and where will be appreciated.
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They're still in good condition. I used some wood offcuts under the base to keep it off the ground by 1/2" to stop rot - so far so good.
Wisdom is not putting it in a fruit salad.