According to the information the Home Depot drain pan is 32 inches at the top and 30 inches at the bottom meaning you would have a few inches on the sides where you wouldn't have coverage. I suppose you could try to shape the sides out a bit for more coverage, but it would be minimal. The best method I can think of is to insert the pan in the grill, mark it where it sits halfway on the charcoal grate, cut along the bottom bend of the tray on the unneeded side of the pan, bend the bottom of the tray up at a 90ish degree angle, fold the sides in, then fold the bottom back down over the sides holding them in place, and folding a small lip at the bottom to make the tray tilt away from the modified middle portion. This way the drippings will run down and away from the side that won't be leak proof because of the cuts/bends. I suppose you could use grill sealer on the tray to make it leak proof but it would only be effective for smoking as it would degrade for regular grilling. Probably about the cheapest solution for a Ranch drip tray.
So options:
1. Cheap DIY is a modified aluminum water heater pan for $44, some cutting, and folding. Not full coverage, but probably good enough if you watch your food placement.
2. DIY with a 2 foot by 4 foot piece of 14 gauge sheet metal costing $100 +/-, then cutting to shape, and hammering into shape. Welding may also be an option, but SS and aluminum welding isn't the same as regular welding meaning your options may be limited for who can do it. You will get full coverage, but the cost and effort isn't ideal.
3. Take it to a fabricator. I may still call a few local guys just to get an idea, but I am assuming it will be in the $150 to $200 range if I am lucky. It may look the cleanest, but for a grill that will probably be use less than 6 times a year it could be excessive.
It might be a bit before I tackle this one. If
@56MPG,
@JEBIV, or
@jhagestad beat me to it feel free to post it here. I am going to update my title so it is easier to find in the future if anyone is looking.