Manhole inserts suitable for deployment in a manhole opening of a manhole structure may include a manhole insert bottom. A manhole insert sidewall may extend from the manhole insert bottom. A manhole insert flange may extend from the manhole insert sidewall. A manhole insert gasket may be on the manhole insert flange, The manhole insert gasket may include a solidified, hardened, and cured gasket material having a synthetic silicone elastomer. The manhole insert gasket may be configured to remain exposed on the manhole insert flange prior to deployment of the manhole insert in the manhole opening of the manhole structure.