I also have a professional fatty. I just switched from doubling up on the SmartPak small hole hay nets to the Hay Chix 1 inch nets – or at least am in the process. I am still giving her two nets at once – one Hay Chix and the doubled SP one.
I can’t quite tell which slows her down more. She prefers the Hay Chix net, so maybe that one is faster, but it’s hard to tell.
I tried putting a single SP net inside one of the NibbleNet style bags and she just gave up, it was so hard.
I recently decided that I would try Juliet Getty’s feeding recommendation for overweight horses, which is basically giving them free-choice hay to reset their leptin-resistance. Over time, they self-regulate their hay consumption and lose weight. It has taken some time – about 6 weeks or so – but she started self-regulating about 2 weeks ago. She’s eating right around 2% of her body weight. She might be getting a bit more – she is turned out with another horse so I don’t know exactly how much she eats in turnout versus her companion.
I will also say that she seems much happier with her food situation in general. She will even occasionally not finish her 2 alfalfa cubes (yes, literally 2) that she gets as a base for her v/m supplement. That would have been unheard of 3 months ago, when she hoovered every scrap of food you put in front of her and demanded more.
So I am waiting to see if we see any actual weight loss, but I have my fingers crossed. So something to consider since your goal is to “fill her up without making her gain weight.”