I dont know if this is a big issue for anyone else but the rear ''bumper'' was a pain for me. It acted like an anchor, scrubbing speed and wore out rather quickly so here is an easy fix i did.
Using 2 AE steering servo mount blocks (b4/t4) mount them in the holes where the rear chassis stiffener posts would be, facing the small indent where the velcro is on the undertray. They are the perfect size without modification, just screw them into the chassis holes with some m3x6 flat head hex screws, aim them at the undertray, mark with a sharpie and put a 3mm hole in the undertray and attach the undertray to the mount with m3x8 cap head screws. Now the undertray is secured without the rear bumper, you can shave it down so it wont drag or scrub speed. You could leave the rear bumper off but i didnt as it keeps the aluminum trans/diff mounts nice and scratch free. You can cut a small hole in the velcro to use the same little indent in the tray.
Obviously if you use the stiff top deck and the rear posts this wont work for you.
This is a pretty simple mod but ill still try to get some pics up later if anyone needs them.
edit: i should have put that this was for the cougar in the title but too late now, lol.