Have you considered the front as opposed to the back?
There are a few IWBs that are designed for wear upfront as well as the "Thunderware" stretch bands that securely hold the weapon in place near your groin/belly.
You would think these are or would be uncomfortable for sitting but I work with quite a few people who carry a medium sized semiautomatic and sit for hours sometimes more than 10-hours without complaints.
Just don't go off half-cocked, it might hurt.