Provided they are kept clean you can use 'above' filters below the lens. I used to do this with my old Durst F30 as the negative carrier was disturbed by removing the condenser/filter drawer unit. I still do it for odd burning in jobs, even though I have a colour head. You can drop the filter over the aperture of a burning card which is convenient. It's a trade-off. Putting anything below the lens risks image degradation, but fiddling with the enlarger can throw the focus off with some models. I need a step to change the settings on my colour head at maximum enlargement, so I sometimes don't bother 8-) Under lenses sets and large above lens sets can be expensive.

Use what you can get now, and get a better solution when and if you can.