Note that restricting airflow on a radial fan can cause the motor to heat up. Make sure you get a 20-inch fan with a fuse. To reduce heat, increase air flow by making a box with three filters and by using 2 inch deep filters. Also, when I built my three filter "box" last fall, I was able to order a set of three MERV-13 20x20x2 filters directly from Nordic Pure for less than what they cost on Amazon. Finally, to make a cube, cut the remaining sides from the filter box. Then roughly tape it together with small pieces of duct tape. When the filters and panels are in the right spots, seal it completely with duct tape. Remember to put the box of filters on the "pull" side of the fan with right sides of the filters facing outwards.
I agree with placing the filters on the pull side, since that is what all hvac units do and what the filters are designed for. To be effective, you are also corrected that a deeper pleated filter with more surface area is the only way to go. But I am unconvinced the motor will heat up. Certainly there will be less air so if it is air cooled it will heat up from that, but at the same time, a fan in a partial vacuum uses less energy and should heat up less. Just slap a meter on it and you will see the draw can be significantly lower, since it is doing less work.
Linked projects put the filters on the opposite side to what you're suggesting. Can you elaborate more on why do you think the filters should act on the air coming into the fan? Is it to increase heat dissipation of the motor?
putting it on the intake keeps the motor cleaner (which will help with motor life), you can vacuum the filter without disassembly, and the air flow will tend to help the filter stay on, instead of pushing it away.
