One fact of home ownership is the very fact that many areas of your property eventually need to be replaced, some sooner than others. Don't forget to measure the top and bottom jambs as well. Sometimes you move into a new house and then realize that a screen is broken or missing entirely.
It is smarter financially and environmentally to lessen the amount of heat you lose instead of shopping for a bigger boiler. Put spacers, 1ft apart, along underneath of the rebate. It still allows the sun in while keeping insects and bugs out.