Summarize:

Using a small stepladder, cardboard boxes, or other temporary structure, place a box fan high enough to at least clear the middle PVC pipe of your paint booth. Position the structure outside of the booth on one side. If necessary because of limited space, place the structure so that it is approximately 1 inch (2.5 cm) from the middle PVC pipe. This will place the legs of the ladder inside the booth, but they will be covered by plastic. Trim a hole that's almost exactly the size of the fan at the height you've got it. Pull the plastic to the edge of the fan and seal it all around with tape. If you need to, add extra plastic to make sure you get a tight seal. You're creating positive pressure, meaning you'll blow air in and let it out in another filter. If you pulled air from the paint booth through the fan instead, you could end up drawing dangerous fumes into the fan's motor. You don't want to blow dust into the booth, so find a filter that's the same size as the fan. Seal it to the back of the fan using duct tape. You can also do the opposite if you prefer. That is, you can tape a filter into the side of the booth instead of the back of the fan. Then, just face the fan toward the filter. Cut a hole in the plastic large enough for a filter on the side opposite the fan. Tape it in place to the plastic, making sure you seal all the edges with tape. When using the paint booth, always run the fan. The furnace filters will eventually fill with overspray and paint dust. Each time the filter is changed, either pull up the existing duct tape holding it in place or cut it out with a blade. If you're using a blade, be careful not to cut the plastic! It may be helpful to add the new filter with a different color duct tape each time so you know which tape to remove or cut without touching the wrong tape.
Create a structure for a box fan. Cut a hole for the fan. Face the fan toward the inside of the booth. Tape a furnace filter to the back of the fan. Mount a furnace filter in the plastic with duct tape. Replace your filters regularly.