hauling a car through Canada

I've towed my racecar in Canada from Maine three times with no serious problems but the last one got my attention. The first two times crossed the border in and out of Canada with no problems, the third time into Canada no problem back across almost a big problem. The border guard asked to see the paperwork for my unregistered racecar, I had none. When I told him so he said "pull over your not going anywhere". Oh great. Luckily the other border guard that was there had seen us go through two days before and after a few minutes let us (a friend had his racecar also) go through. I asked the guard what we could do to avoid this problem again and he said to stop at the U.S. side before going into Canada and fill out some paperwork stating what you have with you and let them sign it. Display this paperwork when you enter back into the U.S. and you should be O.K. This was 2003 and i haven't tried this yet. I may be registering my racecar next time I go to Canada just to be sure. Maybe give customs a call to see what they recommend. Every time I cross the border I hold my breath because you never know what they may be checking this time.