Cut the tenons on the bottom 2 X 4. These will ride in guides on both sides of the entrance.
For the rubber spacers I cut eight feet of oxygen hose off of the couple of salvaged oxy/acetylene hoses I have. They are useful for lots of different diy stuff around the shop, tough, flexible with a thick double wall.
I cut it into 120 - 3/4" long spacers on the radial arm saw. When all assembled they will compress to average out around 5/8".
And here are all of the materials to assemble the door section.