I modified my tool tray, cut the triangle area out and put steel screen in there. Works fine but I also drilled some holes in the air box lid. The air cleaner being a triangle is smaller than the air box cover by enough so as to be able to drill holes around the outside and the air filter is still applicable. I bought seconds of both pieces to replace them or send with if I ever sell one of the bikes. 
the mods worked fine. But, I was able to drill the holes in the air box lid on the second 400 without having to modify the tool box cover. I’m sure people may be concerned about the air box having holes in it but it’s the easiest piece to replace and it’s effective! My 400s are queens so they’re not going to be out in any rain while I own them. 

Lifetime Mikuni experienced mechanic here, my DT400B was blubbering midrange on up.
I found the air passage from the intake horn into the main well was clogged with fragments of the disintegrated foam air filter. Cleaned out, runs CLEAN on up into the redline. 

Thanks for pointing that out & can be easily overlooked.
First time i had that was in a Yamy CS1 180cc twin in about 1968 at work.
Was tearing my hair out trying to sort the midrange on the needle before i found it blocked.
Think i might've started drilling that one side larger before realising but luckily got away with it.

More air flow in the main air jet passage with a primary choke needle jet (vs. air bleed type needle jet) actually makes the mixture richer. 

I'm learning from my mistakes. Still making mistakes though haha. Good ta no theres plenty of scope for mistakes & learning.