Plunging your pots in sand will largely prevent overheating of the soil. Instead of a lidded frame, however, I'd suggest a small version of a lath house. You might not think so, but even such a frail structure will significantly even out diurnal variations in temperature. Make sure the laths run N-S, not E-W. -- Rodger Whitlock Victoria, British Columbia, Canada Maritime Zone 8, a cool Mediterranean climate on beautiful Vancouver Island