I'm a complete newb at this. This is the first plug I've made. Int the picture, the white/black stripe is the left rear quarter window flange of a Jeep Wrangler hardtop. The part will be a box that inserts where the window used to be. Later, a hinged cover will be made for the box.

My question is about the flat area, which is about 25" by 18".  I will be doing the layup with polyester resin by hand, with the plan of ending up about 3/16" thick. The flat portion will be vertical when installed.  I'm wondering if I need to use core to keep it from flexing. If I did, would 1/4" end grain balsa be sufficient? Or, should I put 1/4" ribs in the part that protrude inwards? The downside is the amount of fillets.

I appreciate your thoughts on this.