Mask for 6x6

Hello. Maybe this matter is a frequent question, I've used the searcher but I'm not able to find about it.
I've got an Opemus 6 enlarger. The neg carrier has glasses and I have problems with dust when I place 6x6 medium format( for 35 mm I replace those glasses for metal masks). I've seen that Meopta sells 6x6 masks but I don't know if film will suffer bending. ¿Has anybody some experience?.
Thanks.

It can't be as flat as film clamped in the glass holder, but in reality I've not had any problems visible to my eyes when making up to 12"x12" enlargements from negatives in the glassless 6x6 holder. Not even when using ADOX CHS film, which is notoriously curly.
Ian

What a faster answer!
Thanks Stan.