Strange there wouldn't be a bracket there someplace. I think most states also require a liscense plate light. You should be able to buy a holder at a place that sells trailer or auto parts. I'd check with the DMV to see if a light is required. Maybe not.
Joe & Joan
F350 SD DRW PSD, Cardinal 3825FL
If it is true fiberglass cap, you could possibly use coarse thread screws and anchor right into fiberglass. There is no real load on a plate..although u are probably hesitant to just mount plate to fiberglass recess. Predrill pilot holes...
Eddy & Brenda
2015 Silverado HD 6.0L
2014 Zinger 32QB
They must have left something off at final assembly. All the Cruisers I've looked over either on a dealers lot or in a RV park, had the license plate holder built into the left turn/signal light on the back.
Suggest getting something done asap. Don't know how it is where you are, but down in our parts, a missing license plate is a cash cow waiting to drop some big $$$ in some local yokel's coffee fund.