I agree with the 10-minute block. It may take longer than 10-minutes but worth the effort.
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I am making a double Irish Chain for a raffle right now. Great contrast, very easy, appealing to everyone.

This so nice of you. Go to u-tube and check out the 10 minute quilt blocks. They go together fast and look like you spent a lot more time. I put a top together in less than 3 hours. You can then just pick your quilt pattern. I haven't done my top yet, but plan to do an echo. Good Luck.

I saw this quilt pattern on this board done in Oriental fabric. Here is my version done for the back of my daughter's quilt. The "boxes" can be made any width to showcase fabric. Good luck and God Bless.
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If you want something fast and easy, try doing large squares with maybe sashing in between. You can make them as large or small as you want. I do these when I am donating quilts. They are not fancy, but they are very well received.

I love the Warm Wishes pattern, and can look so pretty and different depending on the fabrics.

Good for you for helping others! I see you have had a lot of good suggestions here, and I add one more. Think about quilt-as-you-go. It's easier to handle, you sew and quilt at the same time, and no expense for a longarm quilter.

I suspect that if you pick the colors, maybe first and second choice, people from this site will help you with fabric.

Consider using a fleece backing. It will cut your expense because you won't have to use batting, there will be a lot of sales on fleece at fabric stores as fall sales begin, and it's a lot less expensive than buying 100% cotton fabric for backing. At @ 58 inches wide you can size your blocks and border to be just under that width, and you can make as long as you want...a great lap size. Lots of people gravitate to the coziness of that backing.

For an October sale, warm fall colors might be appealing. Or something that looks cozy for winter. If you have a nice focal fabric in your stash, Warm Wishes is easy and looks more complicated. Or check out those great tutorials on Missouri Star Quilt Company and use your stash rather than buying precuts. Right now I'm using their tutorial for making a Summer in the Park -- rather than scrappy I'm using 3 different blues and solid white for the strips. It's pretty quick and coming together well...and looks like a harder pattern.

Maybe if you explain the situation to your quilter they would give you a break, especially if you label the back of the quilt and have a photo of the winner with the quilt. Send some photos of the day of the fundraiser to the quilter as proof.

As it happens I have been fundraising this weekend so, without raining on your parade, I am well placed to say that it is hard to separate people from their cash, even for a good cause! Do not spend money on a LAer- you may well not make that much on the quilt. Far better to shop your stash and tie the quilt if you don't want to quilt it on your DSM. A fleece backed lap quilt in autumnal colors with SID could be a winner.