I believe what you are calling trumpets, I call them velocity stacks, the top of each trumpet is tapered(gets larger) as it rises and is flared like a flower at the top, you have six of them pointed upward.
So if you had 6 more trumpets with the bottom cut off an inch from the top each one of them would drop into the top of each trumpet and stay there like you were stacking trumpets(paper cups), so if you have a metal plate sitting on top of all 6 trumpets with holes drilled right above each trumpet that were slightly smaller than the top of each trumpet perfectly centered on each trumpet, you could then take that plate to a press and use a metal ball that sits maybe one third down inside a trumpet, now if you have a copy of the inside of a trumpet flare made out of stronger metal than a real trumpet, you could center the plate hole with the 6 perfectly centered holes above each trumpet over that copy of the top of a trumpet, put the ball on the hole and then press down with the press till it stops, you now have a flared downward hole, do that to all 6 holes and then that plate should just sit on the top of all 6 trumpets like it was made to be there.
You would then attach that oval or square ring you have made to the size of the air filter to that plate you have sitting on top of the trumpets as low and at the angle you need, once done you now have the base of the air filter housing sitting on top of all 6 trumpets, put the air filter you are going to use in position, then you make the top.
I do not know how you would hold the air filter assembly on there. maybe a piece of foam in the cowl will hold it down, but when you lift the hood you can take the air filter assembly off in one piece and it will look like the first photo you posted of the truck with all 6 trumpets standing up in all their glory without anything in the way to hide them.
This is the simplest way I can explain my idea, it may scratch the trumpets unless some seal is used on the bottom of the plate to cushion each hole that rests in each trumpet.