The ornamental design of a barbeque grill bottom, as shown and described.
FIG. 1 is a top front perspective view of a barbecue grill bottom showing my new design, the portions shown in broken lines are for environmental purposes only and form no part of the claimed design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a left side view thereof, which is a mirror image of the right side view thereof;