It is possible to make your own jig if you can get the resistors. There's a bunch of YouTube videos that show you how to do it, but this one is probably the most thorough.
Samsung Galaxy S (How-To Unbrick *USB JIG Method*) - YouTube
I had trouble getting it to work because it's hard to touch the usb mini connector leads when you're just holding the resistors. If you have an extra usb mini cable that you can spare, and you know how to solder, you should look for a video/guide that will show you how to identify the pins and where to put the resistors (I know I came across a few when I had this issue).
I finally got my Cappy into download mode by doing this:
1. Remove battery (I pulled sim and sd as well)
2. Plugged usb cable into pc
3. Put battery back into phone
4. Held down just the volume down button
5. Plugged usb cable into phone
Everything I've found online says you're supposed to hold both volume buttons to get into download mode. I don't know why volume down worked. Maybe it was just a fluke. I had been trying to get into download mode for hours, and didn't really care at that point.