First of all, if she already sleeps loose with no problems and you are happy with that, do you actually need a crate? Not everybody does and as long as you can ensure the room she's left in is adequately dog-proofed so that she can't injure herself or swallow things she shouldn't be eating there's no reason for you to have one just because other people do or tell you that you should.
If you do want her to have one, perhaps so you can contain her safely during the teething phase or you'd like her to be comfortable in a crate for travelling and vet stays, then definitely build up the time she spends in there very slowly and always make it rewarding. By putting in the time and effort now, you will have a calm, happy, relaxed dog at times when she may need to be crated in future.
You're doing everything right in terms of feeding her and giving her treats in there. As you treat her, gradually withhold the treat for longer and longer, starting with a few seconds and building up, so that you are rewarding her for choosing to stay in there, instead of just going inside. Then you can start closing the door, treating, then immediately opening it and do the same, gradually building up the time the door is closed for.
Once she's happy in there you can start giving her tasty chews, stuffed kongs and treat dispensing balls/toys in there to keep her attention build positive associations on being in there with the door closed and build up the length of time she's happy to be left in there. (Tip, don't give her those kongs/chews/toys at any other time than when she's in the crate with the door shut, as then they will be something she looks forward to getting and will therefore encourage her to actually look forward to going in the crate.)
I used a method very similar to this video for getting my pup used to his crate. He loves it from the off for naps and playing with his toys, but only if the door was open.
Now I can do the school run (20 minutes to half an hour) leaving him with a couple of loosely stuffed kongs and a treat toy and he doesn't even notice I'm gone. He also sleeps in there overnight, from 10.30 till 6.00 ish quite happily.