Your options are somewhat limited by the handguards you have, you will need to find a rail section that attaches to your bottom handguard.

Personally, I would swap the handguards for a drop in replacement that is more egonomic and easy to find rails for, like the Magpul MOE ones. These are pretty inexpensive, and have multiple accessory mounting positions on them. A rail on the bottom would allow you to have a bipod, or you could get one that is M-Lok Compatible, like the Magpul one.