We recently added a new feature to the CR-10/10S enclosure: a top slit for the popular modification of switching out the hot end to be direct drive. Although drilling a single hole works fine for most prints, it didn't allow a full range of motion for prints that used the very top of the printing range. For that to work, you need a slit so the filament can move fully from side to side, which is a little hard to DIY. If you're thinking of upgrading the hot end, an E3D Titan Aero has been working great for us.
Although that works great for those who've modified their printers, what do you do if you have the stock configuration? Just leaving the slot there is fine, and you're not going to lose much, if any heat. After all, people with the modified hot end have to leave it open for the filament. But if it makes you feel better you can print this plug that will close up the slit. The part is cut in half, so you'll need to print two. If the fitment is a little snug try printing at 98% size.