I don‘t have a butt plug, but have had a 10 inch x 6 inch dildo inside me all night in a hotel room, knowing that a guy was coming to fuck me there unprotected early the next morning. A dildo certainly stretches the hole more than a butt plug, so it is still quite large after some immediate contraction. I guess the muscle will finally weaken with repetition, making a transgirl‘s hole as pliable and as pleasurable as a woman‘s vagina
Although this size of dildo is intended for insertion into a woman‘s vagina, which gives way more more easily, it does surprisingly slide quite easily into a transgirl‘s tighter hole, if the hole and dildo are well lubed. It appears impossible to push it so far that the hole closes around it, but that would defeat the objective and it could be difficult or impossible to remove then.
One solution, as you have also indicated, is to wear a panty, which does at least prevent the dildo slipping out entirely, so it continues to keep the hole wide open, and can be repeatedly pushed deep back inside, which feels nice too.
In the absence of a butt plug, I also use a rubber-coated tool handle with an undulating shape that ensures it stays in place. I attach shoe laces to the thin metal shaft, as I can‘t resist pushing it in so deep that the hole closes around it all, so need the laces to retrieve it. As the shaft is narrow, it doesn‘t serve to keep the hole stretched. So for stretching it‘s best to have partial insertion.
I advise against using a hollow rubber handle, as when it is inserted with the open end facing outwards, it expands when pulled on a string, making it almost impossible to pull out, similar to the function of the barbs on a harpoon. I finally managed with great effort. Although it felt like rape, I couldn‘t unfortunately get much pleasure from that, as I thought I would end up getting it operated out in a hospital.