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Are you using broth, or oil ?
I would suggest sirloin cubes.
I'm using oil
Flap meat is cheap and tender but is flat thin pieces(unlike sirloin and other cuts that are thicker and can be cut into cubes), so you might want to thread a short wooden skewer through a piece of flap meat. (Beef Tenderloin is certainly tender but it really has no flavor imo.)
We did fondue last year. I used a broth base, but we seared the meat beforehand (I think it was sirloin), then deglazed the pan with bourbon and added that to the broth. It was absolutely delicious!