I would go with silicone and a piece of glass on the inside of the tank with a 1-2" overlap. Bulkheads are also an option but they should be monitored if they are installed as natural vibrations can loosen them over time(several years). I am not a fan of holes in the bottom of tanks as bulkheads can potentially leak and cannot be replaced without draining the tank. I have had personal experience with this when I did tank maintenance and worked at an LFS along with a few nightmare stories on R2R over the years. Both glass/silicone and bulk heads are good choices and it comes down to your preference.