If you fit three plastic blocks inside the top of the lid to the sides using long reach bolts leaving sufficient room to add large thick washers and nylon nuts to the end of the thread to keep the blocks from falling into the tank. Roughen both edges with coarse grit, clean with alcohol rubs, then apply tiger seal. Close up the gap by tightening the nylon washers down so the two sides butt up tightly and leave the sealant to dry for at least two hours the tank should be secure and strong enough to last for a long time.