I built 4 ~4x8 torsion box doors for my workshop last summer and fall. They replaced the roll-up doors in my garage/shop. They worked very well, but here are some lessons learned (the hard way naturally).
The doors are 3-1/2 inches thick. The hinges were attached to the outside front of the door. The pivot point of the hinge is on the outside. The importance of this is that the swing of the door is measured from the front on the diagonal of the thickness of the door, to the back. Thus the door which is nominally 4' wide is actually 4' and about 3/8". The gap between the doors needs to account for this.
The gap also needs to account for expansion. It may be because the doors were installed as the winter weather began and only had the outside and the edges covered with primer only. However the expansion is significant in the very moist weather that we get here in the Rust Belt. In low humidity weather the door is wonderful, in rainy weather I became a madman trying to carve out that radius.
You also must plan explicitly for gaps. That is, the foam or plastic edge sealing strategy that you will employ. Some seems soft but really isn't because the cold stiffens the (EPDM I think) material and may make the door un-closeable. Waste some money now and see how the materials at your local orange or blue box store behave. (They'll be clearing the shelves soon.)
Since I was heating the shop when I was using it and not at other times, there may be some other interactions.
Next of importance is the thickness of the skins. If you are going to have thin material < 1/2 inch, you MUST carefully plan where you will have locks, hinge attachments, handles, etc. You also need to decide whether you are going to use the door to hang coats, brooms, shop jigs, and any other object that might have some weight and be hung on the door.
At any of these points, you must thicken or fill the grid with additional wood. If your grid material is going to be 1/2" for example, You might want to add a 1/2" piece above an below the grid to give you a 1-1/2 inch area you can screw into. And make and keep an ACCURATE layout of where these "thickenings" are located.
Yes it does stay flat (as it expands), yes the 3" of insulation helps keep the shop warm, but don't be casual about planning the door. Once the skin is on and you have painted over your pencil marks, you won't remember where the devil your grid ribs are located, and taking a stab at locating it will prove very difficult and time consuming.