There were, and still are lots of DIY case designs. Templates for cut-and-fold cardboard cases were quite popular in the early days of the Pi, and Lego cases are still a popular choice. However, there are so many inexpensive cases available now that the appeal of DIY cases has waned. It's not possible to give you any precise suggestions, because you never mentioned which model Pi you want a case for, and you didn't put your location in your profile.
Note that all of the single-core Pi models have the RAM package on top of the CPU, so adding a heatsink to those models is not very effective.
My password is the last 8 digits of Pi.