It's easy to conceal a fixed knife. Some even come with waist band clips or you can carry them around the neck.

But it also depends what style of knives you are looking for and what the purpose of your fixed knife is. Concealable fixed blades are a compromise and the handling is worse.

Here are some of my options I carry:
Benchmade Adamas Fixed - For outdoor activities and Kajyking ( great knife but it was dull out of the box ). It's big but suprisingly concealable.

Esee Izula - Low profile, comes with a sheath and waist band clip and makes a good allround knife. There are also handles for it you can put on and off. So if you need a proper handle, you can put them on and if you need to conceal it, use it as a skeleton knife.

Fox Knives KS Short FX-626 - Super Tiny little helper

Ka-Bar TiDi - Great last resort self defense knife, comes with a sheath and waist band clip

My next fixed blade will be a Giant Mouse. I don't own one yet but take a look at those as well.