Bullberry makes them and so does the guys that used to run Virgen Valley , not sure what they are calling themselves now but they make a fluted barrel.
Its hard to convience Fred at Bullberry that you "need" a 300 mag but he'll make one.
Any reason your going with a 24" barrel ?
Not to many guys wll flute the Encore barrels to deep , I assume because they get them finished just ad the lug to it and fluting them might cause accuracy problems if the cuts are deep .
I have a 300 mag barrel 26" full bull from TC custom shop and honestly it realy ain't no fun to shoot , the stock shape on the factory Encores are not setup to handel heavy recoil so they tend to pass it all along to to.
If you dead set on having a barrel made to your specs you can order the balnk from a barrel maker contured and fluted then have the lug welded on and chambered , this will probably be a very costly route and you'll have to do alot of talking to the Encore barrel makers to get them to agree but its a choice.
Also Mike Bellm makes Encore barrels you could get with him and see what he has to say and if he can point you in the right direction. www.bellmtcs.com