Electrical tape, bad idea. Not that there's anything against the tape itself, but the backplate will get warm, the air blown through the cooler will get warm, and the adhesive on electrical tape doesn't like heat. It'll get to be a gooey mess in short order. Small piece of card stock is a much better idea.
Back plates themselves serve 3 purposes. Additional heatsink surface area, limit gpu pcb droop, and look cool. Many are even partially plastic. The most damage you'd suffer is scratching, if the surfaces of the clip and backplate actually touch. 0.1mm is a tiny gap, but still a gap no matter how close they look.
An open sided pc case doesn't have air 'flow as such. For airflow, air enters the case, traverses the gap, picks up warmer gradients, then exits. The same air coming in will exit, in a flow. With open sides, that doesn't happen. You don't get flow, you get replacement. The area by the intakes pushes right out into open air, the area by the exhausts draws air from the huge gap. It's not a flow. So temps end up controlled by the ambient air, making intakes/exhausts next to useless unless the exhaust is directly behind the cpu cooler.
Removing the cover is a good way to test airflow, if temps drop drastically, it's a dead giveaway that the airflow in your case was seriously lacking or wrong, if temps barely budge the you had sufficient airflow.
But that only works really for mechanical cooling, things with a fan. For motherboard components, now there's no real breeze, no replacement of air, because there's no flow. So chipsets get warmer than usual, like Sata controllers, the pcie hub etc. Which can lead to performance issues.
Air cooling a 9900k isn't a perfect solution. The largest aircoolers are barely over 250w capacity, the 9900k stock runs over 200w and with an all core boost to 5.0GHz (very popular) will hit 250w outputs. Minimum recommended coolers are 280/360mm AIO's, with a preference for Full custom loops. 1 good render can easily tax a NH-D15S to its limits. The NH-U14/S fits in just below an average 240mm, the NH-D15 right at/just above a 240mm AIO. By comparison, the CM hyper212 series comes in at 120mm AIO, 140w (ish).