This approach is generally not necessary for relatively small amounts of money invested. Investment earnings, as all other earnings, must be used in furtherance of your organization's tax-exempt mission. No part of any earnings may benefit any private individual except for recipients of the organization's services. Discuss with your broker or agent. Generally, volunteers want this form of insurance before they will agree to sit on your board.
Claims based is more expensive and provides coverage after policy period has expired.
You can buy occurrence based insurance with a trailer to cover potential losses occurring after the policy period. Ask other organizations and your broker. Careful coordination between the person responsible for tracking and thanking donors and the person responsible for recording revenue and making bank deposits will reduce the amount of re-work and contribute to greater accuracy of recording. Any donors to your organization take the risk that your application might be turned down. There are many and some are quite complicated. .