The Elite Survival Systems Stealth SBR Backpacks have an internal length of 20". So if the MK18 is 20" or less with the stock folded it will fit.
This pack does not appear to have a pocket for a ballistic plate, so the plate would have to fit the main compartment. An 11" x 13" plate should fit inside the main compartment.
The STEALTH SBR has dimensions of 20" x 12" x 7.5", so fitment will depend on the overall length of your firearm. The internal compartment has an included adjustable tie-down system to secure weapon in the center of the main compartment. The rest of the compartment is lined with Velcro and PALS webbing, and additional Velcro and MOLLE accessories are available for you to customize it or adjust how it's set up.
Depending on the overall length of your firearm, it may be possible to use this with the AR upper and lower disassembled, using additional Velcro/MOLLE attachments to secure it in place.