The trick in extending top notes is to find a middle or base note that is 'close enough' to the top that the top notes will disappear and only the middle or base notes will remain, but you won't fully notice because the transition was seamless. This not only calls for the right middle and base notes, but also using them in the right quantity, and the ability to at least have a good instinct of what to use to extend a given top note is, imo, one of the qualities of a good perfumer.
You could, for instance, use a touch of linden blossom absolute, orange blossom absolute and some tolu balsam to carry a sweet and thick mandarin opening deep into the heart notes. A common one that is often used is to use jasmine to extend rose, and patchouli, too. Maybe a hint of bay to add a long lasting peppery nuance, and a tiny dash of vanilla and peru balsam to soften the base and add some sweet and spice. Of course this calls for careful balance, but when done right, it really smells like one unified, deep and voluptuous rose note that lasts through the whole of the fragrance. Under the hood it's a carefully coordinated olfactory illusion, but the person sniffing the fragrance doesn't see the smoke and mirrors but only the magic, so to speak.
That being said, I find that small quantites of opoponax are generally not that recognizable in a blend, and do go some length to extending it. I'd say that's the most universal ingredient i've come across, with sandalwood being a close second, but sandalwood is deceptive in its strength. Labdanum Absolute is another.. it's MUCH milder than the oil in the top notes, and so alters the fragrance less, but add too much and it will quickly flatten the heart and take over far too soon.