Several people have recommended the Poise pad, cut in half, and placed in the bellyband. Bellybands are only to hold the pads in place; they don't hold much liquid. They say the poise pads are made for urine, and hold so much more than a regular sanitary pad. I used the regular sanitary pad for a visiting dog, who has a problem with marking, but he was trained to pee outside, and I just took the pad off before he went outside. The proper bellyband should fit snuggly and holds the pad in place. Here's a YT member who sells bellybands.
2Parises Shop Comfortable Casual Style by 2Parises on Etsy Just wanted to say I admire you for what you are doing, I'm glad the little guy found a good home.