Don’t ask us for ideas or for how to build your robot, as this is kinda borderline on this. However, to answer your question, @DavidW built a good slingshot and posted a lot of pics in there.
As for ideas for your robot, FUN RECF has posted a lot of great videos showing brief overviews on various robots at signature events. They posted them all here:
FUN RECF | Over Under Content (scroll down past post 20 or so and the videos start popping up)
You can search youtube for robot reveals and robot explanations. A lot of high school teams love to show what they've built via youtube.
https://www.youtube.com/results?search_query=vex+over+under+robot+reveal
https://www.youtube.com/results?search_query=vex+over+under+robot+explanation