You can use a metal file or even sandpaper to sand around the inside rim of your grill. You shouldn't need to file any more than a quarter of an inch around the edge, and it's okay if filing takes off some of the gasket seal too. More information can be found here.
If you have an SG18, the Crucible will fit, but in some cases you may need to file off the inner rim of the grill to get the Multi-Rack to sit nicely inside. Please also see "Does the Multi-Rack Fit My Grill?" for more details.
If you have an SG22, the Crucible will fit, but you will need to continue using an older-style triangular Cooking Grid Elevator. While the SG22 will fit a Crucible firebox by itself, it has trouble fitting the multi-rack in also, because the SG22 is too short to accommodate the extra height of the Crucible with the Multi-Rack. We recommend purchasing the Crucible for an SG22, but please let us know you own an SG22 so we can make proper accommodations before shipping it out.
Please also see Does the Multi-Rack Fit My Grill?