I wanted a way to mount my Bittydesign AR8-GT3 body on my Arrma Infraction without horrid body clips, so I designed a magnetic body mounting system that works perfectly. The front of the body is held very securely by 8 10lb magnets (4 in the body, 4 on the chassis) to prevent it from flying off at speed and the rear is held by half that for a total of 12 magnets (6 pairs) The rear body magnets are held by a brace which attaches securely to the rear wing mount, and the front magnets are held by a brace which sits inside the front scoop. 

There are two options for the rear magnet posts; one for adjustable heights if you use a different splitter design or simply want more ground clearance (note that it may snap if you use lower infill or standard PLA), and one fixed height one that worked well for me to improve strength. 

The magnets can be found here . They are 3/4" x 1/8" countersunk N52 round magnets.
 

M4 screws are used throughout for exceptional strength and strip resistance.

Later, I may add quad-screw mounting to the scoop mount if durability becomes an issue.

Installation

First, make sure both the factory front and rear body mounting braces are in the low position. Next, once you've printed all the pieces, first get an appropriately sized drill bit (it should slide through the hole in the template without damaging the plastic) and the drilling template. Position the template in the middle of the front scoop and drill both holes into the body. Next, use two short M4 screws to attach the front magnet mount to the scoop mount through the body. The mount should be offset towards the rear of the body (as seen in the photos). Next, remove the 4 M3 wing mount screws from the inside of the body and use longer ones to attach the rear magnet mount in place of the washers. The magnet bulges should be offset towards the front of the body. 

Next, remove all factory body mounting posts from the car and install the magnetic ones instead. In the rear, use the two inner positions. Next, you can install all 12 magnets in their proper location. Because the front magnets are so strong, I recommend putting a strip of electrical tape over the four inner magnets to stop them from cracking if slammed together.