The fan on the bottom of the MonoPrice Mini Delta is a great addition since the initial batch as it really helps keep the electronics cool. But, being on the bottom it will be prone to sucking in dust and small debris, and the punched holes in the chassis are really only good for keeping your fingers out.
This 3mm thick filter frame can hold filter media up to 2mm thick to provide improved filtration with minimal airflow reduction and added bulk. The screw pockets are countersunk as deeply as possible to avoid reducing the already minimal penetration.
For best airflow, the printer will need to be elevated a few mm higher than the stock rubber feet provide. This can be done with common rubber feet, or one of many custom designs here on thingiverse.
This filter frame could likely be used on other 60mm fans, but this piece will not separate the filter from the fan, so that function must be provided elsewhere.
For filter media, I used the "Frost King NF305A" AC filter nylon mesh which is cheap and readily available. Simply trim to fit the octagonal pocket.
I used PLA, it has plenty of strength. You may need to scale by 2% (102% setting in Cura) for dimensions to be accurate on the Mini Delta (I had to). Also setting the "horizontal offset" in Cura to -0.07mm will help keep small diameter holes more accurate.
When assembling to the printer, the power must be disconnected and the bottom plate must be removed. There are metal washers on the inside of the chassis between the fan and chassis which could fall into the electronics. These are apparently needed to keep the fan blades away from the chassis.
Due to the washers and design of the holes in the fan, the standard fan screws provided barely have sufficient length. Do not over-tighten screws when reassembling or the fan threads will strip. Should that happen, a little glue on a toothpick can be used to rebuild the threads.