I mean I’m not sure what your budget is, but a solid roof rack from amazon costs about 120$ on average. Yes you’ll probably lower your fuel economy, but so would all the added weight of a tire carrier. Ultimately it’s your choice but Jeeps are known for a lot of great things, however fuel economy isn’t one of them.
Here’s an example: MPH Production Universal Roof Rack for Truck (Cargo Car Top Luggage Carrier Basket Traveling SUV Holder) https://www.amazon.com/dp/B073LXV6LZ/ref=cm_sw_r_cp_api_i_sL8nCbEPNSG2F