i saw a very nice new bike get dragged 1/2 mile up the freeway after it came loose from its trailer, by the looks of it, after a few hundred yards of sparks and bouncing, it went from an approx value of $4000 to about $2000! so a $20 perr day truck rental is probably worth it.
after seeing this incident, i always use 4 tiedowns rather than 2!
ive done the trunk thing also, but i took the wheels and forks of first so i could stuff it in better, but i wouldnt recommend it!
good luck!