I'm not an expert on t-shirt design, but I believe it's possible to have a variety of designs on hand, with a variety of t-shirt colours, styles, and sizes, and just get them made up as ordered. This might satisfy (almost) everyone, and avoid the need for consensus!

For example, the "Cows" company (based in Prince Edward Island, in Canada) has a catalogue with many different t-shirt designs, and when you order you also choose your own size, style, and background colour. (You can see their website at http://www.cows.ca - it's not musical but it's fun!)

Unlike Peter T., I'd prefer less prominent whiskers (or is this needed for biological accuracy???). Like Shula, I prefer the Arlo Guthrie quote.

P.S., what about mugs & buttons, once you're at it?