Good question. I’m afraid that the answer may involve things that your friends may not like to hear, so you will need to be sensitive.
Your approach with God being the Creator is the right one. As creator he has the right to be honoured and obeyed. In fact there is no other contender for that position. The Bible tells us not just that we should not worship anything other than God, but that there is no other god to worship (Deuteronomy 4:35, 39, Isaiah 44:6, Isaiah 45, Acts 4:12). Every other god is something which people invent for themselves (Jeremiah 16:20, Rom 1:18-25, Acts 19:26, Gal 4:8). Of course God is not going to be happy for his creatures to run off inventing other gods to worship!
Certainly it can sometimes seem that God is selfish to suggest that we should believe in him and no other. But we should be careful when we think this way that we don’t make it seem like God is one alternative among many that exist. God is the only alternative that exists. Therefore it is for our own good that he commands us to put our trust and hope in him alone. There is no other way. (John 14:6, Acts 4:12).
There is only one heaven, and God owns it. The people that get to go there will be the people who trust in the God that owns it and rely on his way of getting there. Jesus is the door to heaven (John 10:1-7). The Bible says that those who want to trust other gods are not choosing one valid option among many, but are in fact simply rejecting God’s way and putting their trust in something which seems better to them, but has no power to save them.
Another approach to take is that of God as the Saviour. God’s saving grace in sending Jesus to die for us demonstrates his great love for us. We are so valuable in his eyes that he would send his Son to die for us (John 3:16-17). That demands a response from us. God has shown us the extent of his love and our only response is to bend out knee towards him (Phil 2:1-10).
Your friends need to come to the realisation that this is the God that we have whether they like it or not. Looking at Christianity is about discovering the God who is there not about making up the God that we want. Ultimately the question that all of us face is “Are we prepared to bow to this God”. My answer is “You would be crazy not to”.
Hope that helps,