I also searched for that cover panel for when I removed the back rest and added a top case. I found that the Givi panel did not completely cover at the front of the panel so there was still a gap across where the seat latch was exposing the frame there. It also does not use the 2 bolts that hold the front edge of the back rest base cover since it doesn't cover that part at all. The seat does cover that bit but I wanted the same coverage as the OEM. So I found the Suzuki official panel available from a number of sources including my favorite source for Burgman parts... Suzuki Parts Nation. That source not only has the OEM flat cover plate but also the mounting bolts, spacers and even the official aluminum mounting plate for the Givi Monokey system. I was about to buy all the parts from there but then someone on this forum listed all those parts and the official Suzuki top case so I bought that instead. Here is the link to that cover plate. Cost is similar to the Givi plate but, as I said, it completely covers the opening.
2015 Suzuki AN650L5 Cover Set Pillion Rider (Optional) | CyclePartsNation Suzuki Parts Nation
BTW- The official Suzuki top case is really a Kappa KN49 case that is re-labeled with the Suzuki logo. Kappa is the budget division of Givi. Made to the same quality but costs less.
Oh, there really was no Executive model of the Burgman 650 from 2013 to 2015 in the USA. There was only the Burgman 650 ABS even though it does have all the features of the PRE-2013 version of the 650 Executive. From 2016 to 2018 there was ONLY the Executive version in the USA and no other version sold. The difference is very slight, the 2016-2018 versions had heated grips and heated seat. There were many that didn't come with the heated stuff but had it added after sale with the factory heat stuff however.