The bubble wrap idea is good... that would create just enough space to allow for air movement and the bubbles themselves would give off constant radiant heat... very keen.
Great homemade greenhouse setup! That had to of saved some money. I bought this prefab posted below for a lil over $5k. I will say I love the 60% UV protected polycarb. Solid in any weather, and the UV protection prevents light stress on those intense summer days =)