Need cushion advice and source.

Hi all. We need to replace the ratty/dusty old wrong-sized futon cushions in our trailer with at least one decent, right-sized bench cushion to sit and sleep on. We've been looking (at cushionsource and other Google-listed discounters) and we think we need a 69"x36", 4" deep double-box-edged indoor/outdoor cushion filled with "cumulus" materials.
For people who have bought custom platofrm cushions, can you recommend a good inexpensive source? And which materials are resilient to mold/mildew, attracting mites, etc. Thanks in advance. -Gary
For people who have bought custom platofrm cushions, can you recommend a good inexpensive source? And which materials are resilient to mold/mildew, attracting mites, etc. Thanks in advance. -Gary