Dan Gill (www.Details4scale.com) makes a centerline tank for the A4 that can be used on many jets where it is the right size, and close to scale shape.

It has an inner tank that holds about 50 oz, removable nose cone, and a scale outer shape. I have one on a Yellow Aircraft F-15C and the extra fuel is really useful. Works well. I learned a lot about the use of very large tygon and fittings, and pressure drops in fuel systems ... it is amazing how the pressure builds up on multiple series tank connections.

I have mine plumbed, going backwards from the fuel pump: to a BVM UAT, then to the two main tanks in parallel (feed and vent lines are paralleled), main vent to the suction port on the belly, and belly vent then becomes the system vent.

I am using the stock yellow tanks and if you fill too fast you see them start to "inflate" from the pressure of feeding the belly tank .. all of which is plumbed with large bore brass (which comes with Dan's tank) and large-bore /316" tygon.

I did do a little bit of reinforcement of the floor of the fuse where the tank pylon attaches, and tied the mounting plate to the formers in front and behind the mounting area. I could imagine a fair amount of aerodynamic stress as well as gravity (the tank, when full, is pretty heavy, and imagine the plane in a steep bank and a not perfectly coordinated turn) so I felt this reinforcement was important.

Really happy with the results.

Dave