Some float spas don;t even have an enclosed frame. Some are like the one in the picture. It's cool only when the room itself is sound proof and there is no light that leaks in. When you are under, you hear every sound vibration.
Some frames are made of aluminum, some titanium, and others fiberglass, and even some acrylic. I prefer the bottom of the tub to be slip resistant. Some are very slippery and when you get up you can lose your balance after being in such a restful state. A good way to tell if the frame is sound proof is to play a radio or something loud to see if sound can enter the float tub. Keep the lights on to see if its light proof, not just not seeing light in a room with the light off.
About half the peopel buy frames with enclosures and the other half want a sound proof, light proof enclosure. Some frames are deeper than others but most of them are about 10 inches in depth.
Also enougher thing to consider is the angle of the top of the frame. Just like in steamrooms the angle is important becuase you want the humidity to drizzel down at an angle and go in a direction to the wall, not drip down and hit you in the face or the eye.
Some frames have to be maintained more often to keep the seals intact and others last much longer.
For those that can afford it, you can send the panels in or even the entire frame to get charged in the chamber and it will permanently hold a charge which can enhance the float experience.
