This is something I've been working on for a while now, and have finally gotten to a point where I feel it's ready for public use.
It's basically a web-based version of Gambo's spreadsheet crossed with C.L. Moffat's Ballpark Creator - simply enter in your park dimensions, wall heights, etc., and it will calculate park factors for use in OOTP as well as generate a picture of the ballpark that you can download and use in-game!
For best results, I suggest you use a recent version of either Mozilla Firefox or Google Chrome - compatibility with other/older/mobile browsers is going to be spotty.
Hopefully this will come in handy for the community - I sure had fun making it!
OOTP Park Generator