I very rarely use other people's designs to print. It has been my experience that few people design things the way I like or with the accuracy I require. You may search for and find the exact thing you're looking for but there's probably only about a 10% chance it will actually be fit for your purpose. The sites where people post designs are loaded with models that are often unprintable or just useless, and are intended primarily for the "designer" to be able to claim he has 1000 designs posted.
If you have a 3D printer, get a caliper and learn to use CAD. Otherwise your printer is going to do nothing but print tugboats and starwars junk, and will soon end up at your next garage sale. Don't use sketchup. It's for architecture and landscaping, not 3D printing. If you want easy to learn and use CAD, try DesignSpark Mechanical. The free version will do 99% of what you'll ever need to make functional parts.
It is not uncommon to go through a few iterations of a design before you get it just right. If you are going to post your designs, post the final one that actually serves its purpose, not the first couple that still need to be tweaked.