Ortlieb Ultimate5 GPS case and the Garmin Montana - Will they cooperate?

[greensie]

I've yet to purchase my GPS device, but as it stands now, I'm fairly certain I'm going to go with the Garmin Montana, probably the 650 model (camera, no Topo maps).  I'm just trying to figure out how I'll position it on my handlebars so I can actually use it while I'm on my bike.  To that end, I'm considering the Ortlieb Ultimate5 Plus and its GPS cover.

The dimensions on the GPS cover are 6.5" (width) by 4.3" (height) by 0" (depth).  The dimensions on the Montana are 5.7" x 2.9" x 1.4", suggesting that IF it fits, it's going to be pretty tight.  I'm hoping someone can tell me for certain that it will fit, go for it, or maybe one of you knows of a better solution I could consider?

Any input is greatly appreciated.