Yeah, I agree with sandman... cardstock is your friend here, especially on a model like this that's going to tend to get heavy very quickly. Also, the off-center weight is going to be more of an issue with this model than similar sized shuttle models that are being sold, IE the Dr. Zooch kit of the same size. The extra weight of the second cargo carrier nosecone, the weight of the cargo carrier body tube, and the weight of the engine boattail will all tend to get pretty heavy on you compared to the weight of the Zooch orbiter hanging off the side, which is surprisingly light, and most of it's weight is concentrated in the wings and nose very close to the ET, minimizing the stack's CG shift off the ET centerline. Basically, you're going to be installing another body tube and nosecone on the side as the cargo carrier, and it's CG will be essentially along it's own centerline, meaning that it will offset the completed stack's CG more than an equivalent sized shuttle orbiter would. The Dr. Zooch kit makes use of an assymetrically weighted nosecone to keep the stack flying straight and counteract the offset CG caused by the weight of the orbiter on the side of the ET-- I'm betting you'll have to do some experimentation and implement a similar system to prevent corkscrewing flights, simply because of the added mass of the heavier cargo pod compared to the orbiter, as well as it's mass distribution. Basically, every bit of weight in the cargo pod you can save is a good thing, because it will minimize the CG shift of the combined stack.
Another mitigation strategy might be an offset motor mount like the larger Estes Shuttle kit used-- offsetting the motor mount in the ET tube to the orbiter side of the tank to put the thrustline through the combined stack CG. This actually is probably the best way to accomplish correcting the thrust centerline/CG problem, as it minimizes additional weight in the ET nosecone... but shares some of the difficulties of determining the amount of offset and amount of weight required, as you'd still have to determine the necessary offset and if any 'tilt' of the motor mount would be necessary.
To sum up, keeping the weight down is the best thing you can do with a model of this type... it's going to be pretty draggy anyway, and performance is limited with 18 mm motors, unless you plan on installing a 24 mm mount or going with composites...
Later and KUTGW!!! OL JR