i tried to build one myself, it went well, but this is not that easy, you really have to try it to see that many things are to take in mind.
but that not impossible.
i get a 3" elbow from my local muffler shop, then i got those flange laser cut in 3/8" 44w steel
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if you have a/c there's not much space, you should remove it.
then the fun begin