You just need to figure out what tools and skills you have at your disposal, and whether you have a friend who can help cover what you need. I've made tops as simple as a plain chunk of plywood with some holes drilled in and the surface sealed against moisture, to ones with the lights built in, adjustable venting and a feeding door. You may even go a little nuts later on and build your own custom enclosure, and have everything exactly as you want it. For now though I would do like Doggy75 says and cover with foil.