With advice and inspiration from RioRico (thank you), I purchased a ProMaster Spectrum 1.7x teleconverter on ebay for conversion to an extension tube. This teleconverter looks remarkably like the Tamron 1.4x teleconverter, but costs less than 1/2 as much (roughly $100 on ebay).
I wanted to use the extension tube with my sdm lenses. which have no aperture ring. After purchasing, i tried the 1.7x teleconverter with my DA50-135 and the screwdrive DA35 macro. It worked with both those lenses, however, indoors, in low light, the DA50-135 would hunt without locking. I also have a 1.4x Tamron teleconverter which focuses in a much more positive fashion, so decided to convert the ProMaster instead.
The tools required to do the conversion was a #000 Philips head screwdriver, available from Sears for $2.95, and a LensWrench for the locking ring that holds the lens in place in the center of the teleconverter. There are available on ebay for $40 to $60. Get the multiple tips models that allow the most flexibility.
After installation of the new Extension Tube behind the DA*50-135, the maximum focus distance has decreased from infinity to about 40 inches max. The thickness of this Extension Tube is 11/16" (17mm). The SDM capability of the lens has been preserved and works effortlessly. One does have to refocus after each zoom as RioRico has said, there is a narrow area of focus for each zoom FL. Its easy to get the hang of, though.
The small Extension tube is lightweight and small to carry. It allows my 50-135 to close-focus and act as more than one type of lens. I'll try it with my DA-300 today and attach more pics of that.