Being able to access the safety on a featureless build can be challenging. The type of safety that will best suit you is dependent on your choice of stock, grip and how you intend to use the rifle. In virtually all scenarios, you will be needing a ambidextrous unit.
If you are using an AtT grip in conjunction with a buffer end stock so your thumb ends up on the left side of the rifle, it is not likely that you will be able to reach your safety selector with that thumb. As such, you want to be able to manipulate it rapidly, you need a right side selector that you can use your trigger finger to manipulate. After testing a variety of safety selectors in this scenario, the only model we have found to have satisfactory functionality is the Strike Switch. At the very least, you want a short length right side lever with a sub 90 degree throw. A full length selector when set to fire mode will extend down too far and prove uncomfortable against your trigger finger. This is due to your trigger finger starting from a higher position with the AtT grip.
If you are using an SS grip or an AtT grip in a configuration that keeps that your thumb on the right side of the rifle, then you have a lot more options. In this case you will be using your thumb to manipulate the safety selector and not your trigger finger. You will still need an ambidextrous safety selector and certain SS grips such as the mega fin may prevent use of a full length lever due to the thumb shelf. In this configuration you should be able to go from safe to fire relatively quickly but not visa versa.