This is a customizable thumbscrew for hex bolts of various sizes. You can set the hex width (across parallels,) hex height, bolt width and the slack factor (I default to 5%, aka 1.05, use whatever prints best for you.)
You can also set the number of protrusions (bumps), their size and the height of the shaft. The shaft height can be important if you need to keep the thumbscrew part standing off a particular distance from whatever the shaft is pressing against.
US hex bolt size chart: http://academics.triton.edu/faculty/fheitzman/hexheadbolts.html
Metric hex bolt size chart: http://www.fairburyfastener.com/xdims_metric_hhcs_coarse.htm
Some of the other designs I've seen turned out not to be very printable in that the shift from hex to bolt required bridging. I also had problems with a weak bond between the thumb part and the bolt part. These problems were addressed by adding a taper internally and externally so that no bridging was required and additional strength was added.
If you print it tight, press fitting after trimming the cavities out works great. If you print loose, a little glue in and around the bolt head will keep the bolt from slipping out. The utility of the part really comes from turning the hex head, so the fit around the bolt is less important.