Least expensive solution would be to staple shade cloth over it, available everywhere.

you could always do some 1X1's in wood to close the gaps. That's how we have ours. then you could do a landscaping mesh material (comes in green, brown, black) meant for sun shade. staple that on top. that's the cheapest option.

Sometimes we use a plexiglass with..as previous poster mentioned..bamboo underneath. The bamboo does weather and change color but it is quick cheap shade.