I can't think of a better Decorative Solar Light material to use as thermal mass than moist soil. The rust colored drums emit energy readily, but the moist soil doesn't give it up too quickly.
Aluminium, galvanised steel or other metal frame requires virtually no maintenance and is also strong enough to take any covering you choose.
Metal frames do conduct heat and cold however and are therefore more difficult to heat and cool.
Plastic frames can only be used with lightweight plastic sheet coverings and are usually only found in small structures likes portable and mini greenhouses.
The four basic choices of covering you will find for a home greenhouse are the traditional glass panes, polycarbonate, fibreglass or polythene plastic sheeting.
There really is no 'perfect' covering or construction material for a greenhouse, and what you choose will ultimately be as much reliant upon your budget as it is upon your requirements.
Plastic sheeting is cheap and the choice of commercial growers although probably not suited to the home user as it is less than attractive to look at, tears easily and typically needs replacing within 1 to Decorative Solar Light .