Unfortunately I think that we're all different when it comes to fit and comfort in our cycling shorts. The only shorts I'm happy with are Pearl Izumi's - I tried on a pair of Shebeest shorts and couldn't stand the chamois. I know there are others out there with exactly the opposite experience.
Another possibility if you have shorts that you like is to add a pair of knee warmers. That will bring your shorts to knicker length, and you can peel the knee warmers off when you're ready. I use Pearl Izumi knee warmers - they are a heavier fabric than the shorts, which is great for me because I use them to protect my knees from cooler temps (below 60 degrees Fahrenheit is when I add this layer).
--- Denise