What do I do if I don't have a roasting pan?

I would just use one of the cheap disposable aluminum ones from the dollar store and put it on the sheet pan to improve handling.

Do you have a 9x14 cake pan? I use that when I'm in a pinch. Alternatively, you could try a large cast iron skillet or a casserole dish. If your duck is too big for any of those, I would try splitting the duck in half and using two pans. Most ovens come with a broiler pan in which case I would use that as a last resort. Or there's always the dollar store for those throw away aluminum ones.

I would use a turkey baster to remove the fat and juices as the duck roasts.