From the picture it doesn't look like there is a lot of surface area for a proper bond between the brace and the rim of the tank. Will adding material on top interfere with the canopy? If so you could brace it from underneath. With the rounded front you're right, i wouldn't use glass. I would use either acrylic or an aerospace scrap of carbon fiber. Both are easily machinable, just don't breath the dust or get it in the tank. I would clean everything very well with rubbing alcohol. If draining the tank is an option I would strongly consider that. If it were me I would not use silicone, I would use a polyurethane adhesive like gorilla glue or a suitable epoxy. Make sure to clamp the tank front to back after you glue the brace and rim. Add the reinforcement piece you have made, glue, clamp in place. Wipe off any excess adhesive. FYI polyurethane adhesives will expand up to 3 times their original volume so additional wiping may be necessary. I would let it cure for a solid week before filling. You don't want all of your hard work to be for naught.
