Printing using different canvas size in LR 3

All -

I'm preparing an image for printing, and the aspect ratio is 1:2 (1504x3008 pixels).  The original was shot at my camera's 2:3 aspect ratio, and the crop to 1:2 'makes' the picture.  I would like a 15x30 print of the image, but my lab does not offer this size option.  Their help forum suggests that I enlarge the canvas size to 20x30 with white space above and below using Photoshop, and then trim the print to 15x30 when I get it back from them.  Since I do not have Photoshop available, what's the best way to go about this in LR 3?

A straight file>export in LR gets me a file at 1:2, and I can't find a way to export with a different canvas size.  Buying Photoshop is clearly the best long-term fix, but I'm not ready to shell out that kind of cash at this point.  Maybe I'm missing something in LR 3?  Does LR 4 offer this option, finally forcing me to upgrade?

After reading some websites, I stumbled across using the LR 3 Print Module to print to a JPG.  This allowed me to set up the 20x30 canvas and generate a JPG file at 20x30 with the white space in the file, and I have the resulting file on my computer.  However, most of the websites cautioned that this approach is not viable for production, creates unwanted color shifts, and some report a loss of clarity in the resulting file due to LR 3's print processing engine.  I can't see image differences between the export to JPG and the print to JPG, but there are enough websites expressing caution that I'm reluctant to send this file to my lab.

Does anyone know an export trick? Has anyone used the print module trick to do this successfully?  Other ideas?

CPLittleton