I've been thinking about that lately because both were built over the past several months. When building the Corsi box, I bought a six pack of Merv 13 filters off Amazon for around $80. They are white - or they were white. As I looked at the build photos, they are a bit brown now, so at least they are working. I'll probably replace them at the six month mark, which will be by the end of summer.
The near-field, I'll probably change the pre-filter this week since it's been working for about six months.
If you're thinking about building your own air filters, The 3D Handyman has an excellent series of videos on different approaches to DIY filtration: