reflector, socket assembly.
For the reflector, you can buy one from an hydroponic store or home hardware, it's like 20$ vs 50. Same thing.
If you don't want one, you can use mylar if you have a top ( i assume you do if you use DIY mh ) but if the distance between the bulb and the mylar is too close it may melt. Mylar = 1$ per feet, 99% reflection.