Is this different to the footmuff? Is it just a liner to sit on but doesn't cover up the child? If so I think you dont need one. We have a footmuff for colder seasons (which lines the seat also) but in the summer we leave it off and just have the basic seat.
My friend gave me our footmuff - her friend bought a pink one but then when baby arrived it turned out the gender prediction was wrong at the scan, so off she went to buy a blue one. She fave it to my friend who doesn't have a bugaboo and she couldn't get it close to fitting her pushchair, so she passed it on knowing we have a bugaboo. Long story but I mean to say I think the fit is probably pretty specific, I think if you've got a bugaboo only bugaboo accessories fit. I got a "fits all" drinks holder once that didn't fit the bugaboo.