Seeking Advice on Long-Term Incentive Plans for Key Employees

Hello all,

I have a key employee who has been with the company we acquired for a very long time. This employee has only been compensated with a salary. This past holiday season I began offering a discretionary bonus. However, I would also like to establish a long-term incentive plan for this employee to retain them with incentives. Equity is out; but cash is the play. My thought is to offer quarterly bonuses based on achievable performance targets. These quarterly bonuses would be placed into a dedicated account for this employee and unlocked for them to access after the vesting period. But instead of simply establishing a holding account, I would like to allow the employee to invest that money so it grows over time (the vesting period is 10 years). Has anyone ever done something like this? I've researched 401k plans and that will not work as I need an incentive plan for a single key employee.

Thanks in advance!