Hi Mary - poor Lucy, and good on you for keeping her comfortable. A few things that may help.
For the diaper itself: washable cloth diapers with a disposable liner tend to stay put better on stocky bulldog builds than all-disposable ones, and they're kinder to the skin for all-day wear. Measure her waist right in front of the hind legs and go by the brand's size chart rather than weight - at 42 pounds she'd usually be a large, but the waist measurement is what keeps it from sliding. If she wriggles out of them, a pair of diaper suspenders or a onesie-style cover over the top makes a big difference. Whichever you choose, change often and give the skin time to dry - urine scald sneaks up fast on seniors.
One thing worth mentioning to her vet if you haven't: new accidents over the past month in a senior girl isn't always just age. If Lucy is spayed, hormone-related (spay) incontinence is very common and often improves a lot with inexpensive medication. A quick check also rules out a urinary tract infection, which is easy to treat. If it does turn out to be simple incontinence, diapers plus medication together usually solve the daily-accident problem.
Hope you find a setup that works for her.