You can buy kits to relocate mirrors, but then you've got to switch back and forth or have mirrors that you can't see when the doors are on.
I devised some easily removable mirrors that do the trick for me on the days when I'm running sans doors.
Please note: I cannot guarantee that these mirrors would be 'street legal' and prevent you from being ticketed for not having sufficient mirrors.
Step 1: Materials
Mirror (purchased at Wal-Mart - this one was $2.60. Be careful not to get one that is too convex or everything will appear too far away. You could use a flat mirror as well here)
Metal Straps - these are typically used in deck building. Very cheap at any hardware store
A pair of scissors is helpful as well.
Step 2: Bend One Metal Strap to Fit
Bend the strap to make a 'Z' that should fit loosely in the space between the window pillar and the speaker.
Step 3: Bend the Second Strap
This gives plenty of surface area to tape the straps together and makes the structure more solid.
Step 4: Tape Together Just in the Middle Section, Then Cut the Sponge to Fit.
Step 5: Confirm Everything Fits
Before going further, 'dry fit' the sponge section in between the window pillar and the speaker. Bend the straps to the shape you will desire eventually, paying most attention to the part behind the speaker. It should fit to the ridge seen in the picture if at all possible, which will keep the straps from moving around in the wind.
Step 6: Attach the Mirror to the Strap Using the Adhesive Backing and Install
These mirrors come with adhesive backing of it's own. I cut the backing to fit just the strap, so when I only remove the backing where the strap will his. This keeps the rest of the backing from picking up debris and otherwise looking dirty.
Once this is done, adjust the mirror as necessary and have fun!
Important note: As mentioned, a convex mirror makes things look farther away than reality. A car that looks far enough behind for you to move over to their lane is likely RIGHT behind you if you are using a convex mirror. Get comfortable with judging distances before you rely on this for changing lanes.