I believe you should be able to fit a culture plate with no lid. However, you may need to utilize a different objective. A flask presents more of a challenge. Just thinking out loud here… You may need to build a taller condenser tower and perhaps modify the light source to be stronger, or you may use a shorter flask altogether. A chambered slide should work as well.
I have no experience working or building a fluorescence microscope with the OFM platform. But it should be possible.