Nice build! The 2 exhausts in my opinion work better to prevent the heat escaping too quickly up a shoot. Though your exhaust vent pipe length might still cause it to "suck" out. It'll be great, just will burn more coals than an average UDS.
Regarding something to diffuse with: Don't stress it. Run it like it is first. When you put big cuts that stay for a long time, try putting the FAT side down if doing a brisket to act as a shield from the direct heat. Pork butts I do fat side up because they are just a lot easier to cook.
But the thing I've done multiple times now (meaning multiple smokers I've built) is to put bolts down fairly low to put a cooking grate on. Down at that low I might smoke a whole chicken because it works better at higher temps. But when doing something else, leave that grate down there and just put something on top. Perhaps some AL pan but punch holes in it. It's most important to let those juices drip into the fire. What I do I on top of that grate I lay down some angle iron (or angle AL) pieces close together. It allows the juices to drip into the fire, but the main heat is forced to come up out through the sides. It helps stabilize the temp throughout the drum nicely.