I have tried making a web app — CfLockout that allows you to create practice contests of Codeforces problems. You can select different ratings of the problems and only the problems which are unsolved by all the participants appear in the contest.
Only the user who solves the problems first is awarded the score, so be quick to solve the problems ;)
Note — Email verification mails may lend in your spam folder, mark them not phishing to be able to click the verification links.
Do star the repository if you like the project.
Update #1: The maximum number of problems allowed has been increased to 10 and the maximum difficulty of the problems is now 3500.