Custom moses basket mattress?

Basically my MIL has given us a Moses basket that has been in her family since her mother was a baby, so naturally she wants the next generation to use it. I already have a basket I plan on using upstairs with me but I plan on keeping the other one downstairs to save me moving my current one around. The only issue is that the base is quite small (I reckon baby would only fit in it for a month max) and the sides aren't covered with any kind of sheet. To please the MIL I'd like to at least use it once and show her so she doesn't think I've let it sit and gather dust, but I have no idea where to get any custom made mattresses from or any covers to protect the sides??