On one of my tanks I use a scrape siding that is cut to snuggly fit the inside lip of the tank. Then cut two circle holes to snuggly fit the lighting fixtures and added some metal grading, use to cover gutters, on the bottom of the light fixtures to hold the lights. This was cheap at Home Depot, siding scrape $0.71(where is the cent symbol on the keyboard), metal grading $7.92. I even had Home Depot cut the scrape siding to the size I needed and it was free. If this doesn't work for you, you're out less than $10.