I normally use strips cut from lead roof flashing, and also either lead shot fishing weights, or air rifle pellets. These are 'stuffed' in position, and held in place by a lump of 'Plasticine'. Once you've got as much as you can into the nose gear bay, either fit, or dry-fit the wings, and balance the model on your finger tips at each wing tip. If it doesn't look like there's enough weight up front (ie, the model doesn't dip nose down!), then you can probably get some weight as far forward in the engine nacelles as possible, without interfering with the engines themselves. As long as it's forward of the main gear legs, it should work.