Hi zizibee -- not sure if this is the same basic "ottoman" style as your footrest, but see if this helps any:http://www.upholster.com/howto/slip2.html
Assuming that's pretty close to the style of your footrest, & if you don't actually need anything that formal or tailored looking, just a cover -- you could play around with another idea I've seen before:
Drape a big round or square piece of fabric over the top of the ottoman. The fabric edges need to reach at least one-third to one-half of the way down the upholstered sides (in other words, fabric edges need to reach fairly-well-past the open-ish part where ottomans 'pillowtop' meets upholstered cube part). Fabric doesn't even 'have' to be hemmed, unless it's the ravel-y type ----
Then pin the fabric to the upholstered sides, pleating a little here & there as needed to fit it as desired (either heavy-duty T-pins or the curly pins normally used to fix a throwcover in place would work, depending on fabric weight/thickness) ----
Then make a simple elastic-top skirt (similar to bedskirt or elastic-waisted woman's skirt) to pull over the ottoman. The 'skirt' just needs to be long enough to cover from a little way above the pins down to near or at the floor. Could either be full/ruffly or simpler & more close-fitting, as desired.