My solution to cold frames was somewhat different. Due to severe back problems, I could not get down to ground level, so I decided to build them upwards. I would really call them raise beds.
The first one was made from a breeze block structure using 3 pillars as the base. Across these I put reinforced concrete lintels to support the base made of 3 x 2 concrete slabs.
The height was made at 30" plus whatever frame height you wish after this. It has been up for over 20 years and is as solid today as it was when it was made. However, it is advisable to have help moving the lintels and slabs as the weight is considerable. I had to do it on my own and it hurt !
As I was running out of space, I then built another raised frame from 3' x 3' fence posts and fence rails - all rot proofed. The bottom was made from 21 mm thick rails and covered with a waterproof membrane. I have not bothered with any frame on top as more tender plants are brought into the glasshouse for Winter.
I am glad that I did this years ago as it is beyond my physical capabilities now. It makes a huge difference being able to "garden" at waist height without the bending.
Another advantage of these raised structures is the space for storage underneath especially for Spring crocus and narcissus in pots which are perfectly hardy and can be brought in for flowering.
