Last year I tried Early variety at bottom of plastic 4 tier greenhouse. I used the old pre wheely bins recycle boxes which are about a foot deep and 18 X 12 inches. You can buy similar boxes. I sowed one in Jan and one in Feb. Both matured about the same time. In Mar I took the boxes out of the greenhouse and had a nice supply of early carrots. This year I have an allotment and a proper greenhouse and am doing the same. By using boxes it means that they use greenhouse space (unheated) when it is not needed for other things but can be taken into the real world just as the space is needed for less hardy things that need hardening off.
It is a win win situation.
This year I plan on putting some lettuce, radishes and spring onions in containers in March/April, they can come out in May in time for the peppers to be planted in the greenhouse.
Will it work? I don't know but if it does I will have salad stuff to use when the weather becomes "salad eating weather" and not take up much needed greenhouse space.