I built myself one a couple of years ago, I'm pretty handy with joinery etc . I was lucky enough to find a conservatory for sale , so I just butchered it for its uPVC doors and windows.
Look out for any kingspan sheets on marketplace,you can glue them directly to the floor and do the same on top with plywood ( min 12mm) and that will make the world of difference. If you ever get the chance to put a decent floor in , don't use laminate it's fucking freezing