Way Motor Works has the very useful and affordable CG Lock in stock.

The CG-Lock clamps to the tongue of your daily driver's existing 3-point, factory installed seat belt. Once installed, pulling up on the shoulder belt tightens the lap belt portion "racing harness tight" - keeping it as tight as you want until you release it. The shoulder harness remains unrestrained as normal for maximum arm and upper body movement.
By providing you with about 80% of the holding power of a full racing harness, the CG-Lock (the "CG" stands for "Center of Gravity") prevents you from sliding in your seat. The CG-Lock lets you keep your feet and hands light on the pedals and wheel resulting in better control, faster reactions, and less fatigue. Becoming "one with the vehicle" helps you feel the road and the car's "balance", enhancing your driving abilities.

Thanks
WAY