Sail cover
Unfortunately, your chances of getting one that fits properly are slim to none unless YOUR boat with YOUR mainsail, stacked and furled on the boom in the way your normally do it are physically measured-hopefully by your canvas shop or sailmaker with a canvas dept..
If you are in an area without any good marine shops or sailmakers nearby, all of the good ones can email or fax you a measurement form, and you can take the measurements yourself and send it to them.
While the most important number is the length (E + distance around the mast with the sail on the boom), there are number of other measurements around the boom at a few locations, around the mast, etc. that are the key to getting a good fit. Most of the forms are self-explanatory and show you how and where to measure.
If a vendor tells you he can do it with the just the E dimension, or says he has the dimensions on file, you are taking a big chance that the dimensions he/she has are not the same as yours-a new mainsail stacks higher than an old soft one, for example-the number of luff slides, whether or not you have some kind of Harken or Strong/Tides Marine mast slide system, or Stackpak are all key factors...even having lazyjacks makes a big difference, and no 2 installations are exactly the same
Get it measured by the person doing the cover, and any problems are THEIR fault-do it yourself and you are responsible for the accuracy of the measurements.
Best choice: Have the supplier measure it
2nd choice: Get the form and take your dimernsions and send it to the supplier
3rd choice: Take your chances with an off the shelf or vendor supplied (but not measured) cover.
Any questions, let me know..
S