Need to make cycling pants

[sparkyk1971.livejournal.com]

I have recently joined a gym and begun spinning (cycling) classes.  I enjoy the class very much but I am bruised from the bike seat... and padded cycling shorts/pants are very expensive.  It seems I should be able to use surplus flannel and sew padding into a pair of well-fitting (not too tight or too loose) sweatpants.  This way I can make sure all strategic areas are covered.

I checked the memories... has anyone ever done this before?  Any advice?  Thanks!