Filters don't need to have a “tight” seal, but they should be easy to install and remove, and not so small that they move freely within the unit. Thicker air filters may seem like the best option, as they tend to last longer due to their increased square footage for capturing and retaining air particles. A 1-inch air filter may need to be changed every month, while a 4-inch air filter can last up to six months. Cutting an air filter is possible if you know what you're doing, but it can create air gaps and allow contaminants to pass through.
Cut-to-fit air filters offer the same benefits as regular air filters, with the added bonus of being able to be cut into any size or shape. Oven filters must fit securely, but no need to force them into position. If you have to force the filter into its slot, it's probably too big and can weaken from the cut. If an air filter is too large, you should return it instead of cutting it.
Forcing an incorrect filter size on a filter can cause it to bend, damage it, or reduce its ability to function properly. Filters are smaller than their slot to allow easy replacement. Some HVAC units may need a filter with unique or unusual dimensions, in which case you will need to order a custom filter. It's best to have a little wiggle room in the slot of the HVAC unit, so you can easily slide the filter in and out without it bending or breaking.
If the system can't get enough air through the vents to cool the rooms in your house, that forces the air conditioner to work harder. Even with regular maintenance, leaving a dirty filter inside the air conditioner can cost you a penny in repair bills and even shorten the life of the unit. HEPA air filters are always the most effective method of preserving air quality. If you rent or live in a place where you can't install normal return ventilation, then a cut-to-size air filter is your only option.
So, if it fits your oven, then a 4-inch air cleaner is better than a 1-inch air cleaner when it comes to improving air quality.