You would want to get a configuration that is close to what he wants straight out of the box... otherwise it is a waste of money.

For example, Jagular recommended one in the post above. But if your husband wants a quad rail or prefers Key Mod over M-Lok, you wasted your money paying for a rail system that he is going to replace.

I'm sure some will recommend the Sport, since it is inexpensive... but if he wants a free float handguard, he will have to remove the flash hider and replace the A2 gas block (front sight) with a low profile gas block to get the rail system to fit.

There are just too many variables for anyone to recommend one without knowing exactly what your husband wants.