I have seen these retro suitcase up-cycled stool all over the internet for a while now and have always wanted to make one. The following 'able' is my attempt. Now, I dont know about you, but I never make something perfect the first time and when I try this again I will make some changes. I would defiantly use wooden legs with built in mounting bolts rather than how I did the legs with this project.
But I had to use what was available at the time...
I wanted mine to open for storage and it needs to be strong enough to sit on.
The result is OK I think.

Materials:
Retro suitcase of your choice
12mm MDF sheet
20mm diameter aluminum tube (wardrobe rail)
Mounting brackets
20mm rubber feet
PVA glue
Epoxy resin
Screws, staples
Cushion

Tools:
Jig saw
Sander
Cordless drill
Ruler, tri-square, tape measure
Hot glue gun
20mm hole saw
Hack saw
Clamps

Step 1: Gut the Suitcase.

Step 1 is to prep your chosen suitcase. I want to be able to use the suitcase as storage so we need to carefully remove the internal upholstery.
Carefully lever the staples and remove all the upholstery.

Step 2: MDF

I wanted to add considerable strength to the suitcase lid so that someone can sit on the stool and to the base in order to secure the legs.
I eyeballed the MDF and cut them roughly to the right size.
Next I sketched in the curves for the corners.
Cut the corners with a saw.
Rounded the corners with a rasp and sander until they fit in snugly.

Step 3: Base Structure

Because I want to be able to sit on the stool, I decoded to make 2 side walls to take the weight of the lid.
I measured the depth of the base and the depth of the lid to find the height of my side walls.
Yours will, of course, depend upon your individual case.
Glue, clamp and screw/nail them in place.

Step 4: Legs

Now I used what was available to me. I up-cycled a wardrobe hanging rail. Its a Aluminum tube and plenty strong enough.
The red 'feet' I picked up from a local hardware store for about $1.
Cut you legs to length with a hacksaw. They will probably not all be the same length, (mine were not...) so use the shortest on to mark the others and file them to size. Mine are 25cm ish.

Step 5: Lid/Seat

I wanted to add a little wadding to the seat to perhaps make it a little more comfortable but to also to give the lid a nice rounded shape.
I sacrificed a cushion off the couch (don't tell the wife!) and sandwiched the wadding between the lid and the MDF.
It was pretty complicated combination of hot glue, epoxy resign and staples to fit firstly the MDF and then the upholstery back into the lid.
I mixed up a fair quantity of epoxy resin and spread it around the edge of the MDF.
I then quickly added a few 'blobs' of hot glue and pressed it into place, holding it until the hot glue cooled.
That should keep it in pace while to epoxy does its job.

I could have done with a third hand during this step!

Step 6: Fix the Legs.

This is the part where I REALLY wished I had been able to find wooden legs off some old coffee table or something.
Would have been so easy to just drill and bold the legs on!
However...here we go....
Firstly I marked out where the hole should be drilled and then using a 20mm hole saw, drilled through the base of the case.
Then I glued in the MDF base and left it to dry.
Then I marked the hole saw about 5mm from the end. Drilled down into the MDF to make a 5mm deep ring to fit the tube legs into. Tricky to keep the drill perfectly vertical. A bench drill would have been better.
Because I used wardrobe rails for legs, I used the mounting brackets to fix the legs onto the MDF base. There are small grub screws that attache the mounting brackets onto the legs.