If you have a saw, a wooden lid wouldn't be very hard to make.. or course that won't help with the light.
Congrats on getting them a bigger tank though. Goldfish are big polluters and need a lot of room to... not die.
I only have a couple suggestions for you.
1. you should try to find a better filter (they really are big polluters)
2. if you need some natural looking gravel home depot sell large bags of pebbles for less than $5 (it was enough for my 50gal)
3. If you wanna be crafty get some aquarium sealant, some rocks, and make a bunch of little caves for your fishys... and if you want to make some decor yourself just grab some clay at the craft store and get it fired up at your local pottery store.Also, if you are really ambitious http://www.cichlid-forum.com/articles/diy_aquarium_background.php
If you ever need some fish care advice check out fishlore.com