Your situation is a very good one, and the concern about tax is valid, but it is important to understand how it really works.

First, the money you contribute from your salary is not taxed again. That part is already after tax, so there is no issue with double taxation on the principal contributions.

However, the interest your cooperative earns from lending money is considered income. That interest is what may attract tax under Nigerian tax laws, because it is profit generated from an investment activity, not your salary.

Let me Explain:
Imagine a group of traders contributing money together to lend to another trader at interest. The money they all contributed is not taxed again when returned to them, but the interest they make from lending the money is like profit from a small business, and profit is what tax authorities may look at.

Now, regarding structure, it is important that your cooperative operates with proper documentation and possibly as a registered entity.
Many staff cooperatives register as cooperative societies or formal associations so that they can operate transparently and manage tax matters better.

The best approach is to:
Keep clear records of contributions and interest earned
Separate contributions from profits
Consider registering the cooperative officially with CAC if not already done
Consult a tax professional or accountant to understand how to correctly declare or manage the interest portion

Contributions are not the problem, but the profit earned from those contributions is what may attract tax, because it is treated as investment income.

So the key is proper structure, proper record keeping, and professional guidance to ensure compliance while continuing your cooperative activities smoothly.