I don't know anything about making one and I would be hesitant about doing so. The covers that are sold with the seats or as replacement covers by Britax are crash tested and meant to be used with the seat. Making or using another brand of cover will likely void any liability Britax would have if you were in an accident and your child was injured. If you want to go ahead and make one I would call Britax first and ask about it 1-888-4BRITAX.