Rena from Barkin Dog Clothes made my little Dixie a diaper with suspenders so she can't get them off. She custom makes them to fit just right and they are better quality than the ones in PetSmart and about half the price. Dixie is in hers a lot because if I'm not watching she will pee on my floors. I put a mini pad in the diaper and she's never complained once about having to wear it or tried to take it off. She's able to move freely. It's not bulky at all. An added bonus is that instead of peeing in the diaper she is now coming to me to tell me she has to go out!
I think it would be perfect for your little girl while she's in heat. Give Rena a shout.