Much cheaper than a rear extention you can get a 'top hat' lens board.. the opposite of a recessed lens board.. I'm not sure about your model, but reversing a recessed lensboard may be the equivilent.
But it may not even be an issue if you can focus as close as you want to.
I was looking for one of those, anyone have a line on where to buy one that will work? My board is 110X110mm. I didn't see one after about 20 min of looking on the net. I did email skgrimes to ask if they could fabricate one or a cheaper extension for me, waiting to hear back.