What you propose seems to be a "Cold frame"

What ever size it will be to small. Why not lay out a few seed trays and
pots that you think you want to use in it and see how many fit.
The idea of a sloping roof/top is
A/ to let water run off, and
B/ to catch more sunlight in the winter when the sun is at a lower angle.
I doubt if you will make it airtight but ventilate as needed in daytime,
and remember you can shade it from outside to keep some of the heat away,
and in the winter you can always cover the outside with sacking or something
to keep extra cold out.


David Hill
Abacus nurseries
www.abacus-nurseries.co.uk