Actually, that's a pretty standard size for a residential system. That said, there's no real way to do this, and it would not be financially a good idea if you could. If the furnace is in the home there are ways to put the filter right in the bottom of the furnace itself, but otherwise, I hate to say it, but it would be difficult to do much about this situation other than to call a company to do a semi-annual maintenance for you. I understand the inconvenience, but again, it would be difficult or expensive or both to rearrange this. A cheesecloth over the vents would be an option but would be a lot to change them regularly. It wouldn't catch all that goes into the system, and you would want to remove the main filter. I would even more so recommend a servicing once or twice a year since the blower will likely dirty up faster so you might as well just let them change the filter and save the other efforts.