GLASFLOSS
Glasfloss GTASP18182 Panel Air Filter, 18 x 18 x 2 Disposable MERV5
You will find that 1/4 in is thin enough to fit but thick enough to last.
Glasfloss ZLPSP8301 8" W x 1" D x 30" H Pleated Air Filter MERV 10
The AQBB brand gives you a good balance of strength and weight.
Glasfloss 12244-M10 12x24x4 Pleated Filter MERV 10 Air Filter
The HBN 0.5microns unit holds up well in environments with high particle loads.