There are slim filters on the new & used market.
These are/were marketed for use on wide angle lenses, or for lenses with slip-on lens hoods and attachments..
Not as common as others and usually quite costly when compared to other regular filters.
A neat trick, due to the close physical proximity of the (Siamese) Twin Mamiya, 'C' lenses, is to install the filter of choice on the upper viewing lens....first.
Mark an index spot (top-dead-center, for example), on that filter and the outer lens barrel, then remove the filter.
Now with the filter removed, file away a flat, or a portion of the outer ring of the filter ring, at the bottom most point of the filter ring, a point that will be directly above the Siamese top of the lower taking lens.
Now, when the filter is re-installed on the upper lens, and aligned to the index mark, a filter, any filter(?) can now be installed on the lower lens.
Your choice? How many filters are you going to file/modify, to be used on just the upper half of your Mamiya TLR lens sets? For most, just one per lens set! A clear or a minimal skylight/haze for protection...that's it.
A lens set with two filters attached in this manner, usually will not accept the standard Mamiya, front lens caps or hoods.
An indexing method, similar to the method above, is employed for using polarizing filters on TLR's, too.
You first attach the polarizer to the viewing lens, mark or tape the two rings of the polarizer at the desired effect, then carefully unscrew the polarizer from the upper lens and move it down to the taking lens. Click n'repeat as necessary!