04 Honda CR-V headunit custom kit

HAving some issues installing a new sony headunit into my 04 CRv. I am using MEtra custom kit # 99-7898 and none of the side brackets included with the kit MAtch up with the factory holes on the car. The factory stereo has some metal L-shaped brackets, but there doesnt seem to be any god way to secure those to the custom kit either. Any ideds? Thanks in advance!

Also, where would be a good p[lace to install the amp? I was thinking under the drivers seat, but there isnt much flat space under there.

i would install an amp anywhere away from moisture,dirt,dust and hot places .Under the seat has a tendency to overheat amps especially if they dont have a built in fan.Back in the day ive installed them under seats but not in a long time .Put it in the trunk or the back of a fold down seat .Im not saying you cant put it under a seat ,you definately can hide it  but you can risk overheating .

    Alot of new cars( Hondas for sure, I've been a tech w/ them for six years) have HVAC ducting under front seats to blow heated air on rear passengers feet. That means hot air right on your amp!  Put it on the back of the rear seats.  You can buy a factory cover from the dealer that will cover the whole rear storage area, hiding your amp, subs and any other equip.  Stay away from the floor in the rear the gas tank is mounted right to it, wouldn't be the first time I saw amp screws go into them.  Sorry, no help w/ the install kit