Please don't mass PM users who happen to have a Forma listed in their device info, that comes off as a bit rude

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That said, I'm using, as usual, a $5 neoprene sleeve. Absolutely not adapted to the Forma's form factor, but, eh, that was an early purchase, and it was cheap. Does what I need it to (which is essentially ensuring the case gets a fine printing of dust instead of the device, and being able to throw it in a bag without a second thought), though, so I'm happy.
I've never cared for the sleepcover feature, and I'm a "naked device" kind of guy. The only devices which have a cover with a specific feature builtin is because that feature had a significant appeal to me (i.e., a light, as my K2, K3 & Kindle Touch covers do).