Well, you're making it very hard on yourself with the "open triangle" and flat seat. An under seat tray is your best bet:
http://cognitomoto.com/collections/electronics/products/under-seat-electronics-tray-shallow-cb350-cl350See if Devin (owns CognitoMoto) has a tray for a 450 already. He's got a 450 frame sitting in his shop, so he might be able to make one for you quickly.
Once you start the "reduce, relocate, minimize" endeavor on these bikes, I urge you to replace the stock electronics with a MotoGadget M-Unit, and a solid state Regulator/Rectifier. This reduces the number of components, and their size tremendously. It also allows you to re-wire the bike with a simplified harness, and everything becomes clean, stable and reliable.
Here's a 350 with basically no room under the seat despite a very small cowl that used this exact system:
You can see, the battery had to be oriented here in an unusual fashion, above the m-unit, with the solenoid tucked beside it. Of course, you could move the battery and provide some extra room on yours. This bike was super clean in terms of "electronics" hidden away. Even the Reg/Rec was hidden inside the frame tube at the rear of the motor, under the seat post.