On the tailgate:
Take 3” (aluminum preferably) flatbar and cut to the width of the opening. Then weld on tabs at the ends and one in the middle that went down between tailgate and bed so it can’t slide forward or backward. Then take 4 pieces of double sided velcro about 8” long and siliconed them to the inside wall of the bed above the wheel well. You can store the thing over the wheel well, loose no space and it’s always in the truck yet out of the way. Everytime you needed it, it’s there. Problem is - its so conveniently out of the way and out of sight (spray painted to match bed) that when you sell the truck ya might forget to take it out. Wanna ask why I know that part? Haha.
The aluminum is just so it’s easier to secure with the velcro. You can obviously use steel just might need a couple extra straps.
The siliconed velcro straps are easily removable so it all comes out clean when selling the rig.
On the cover… have you ever done mpg comparison testing? Have it rolled up or even off then added it and tested yourself? I know some trucks they claim it makes improvements and some not. I remember mythbusters doing tailgate up and down testing and showed that on some trucks tailgate up and empty bed actually improved it. But they did do gmt400 iirc.