Thanks one and all. This is all very useful.

I had also planned on extending the height of the standpipe by a foot
or two (there is a shelf above which limts how high I can go) but I
stopped when I thought the current fix might be sufficient. My plan
is to get a piece of PVC the same diameter as the (metal) standpipe,
put rubber gasket material, and perhaps harder plastic or aluminum or
something around them above and below the joint, and hold the upper
and lower pieces in place with hose clamps. Sound reasonable?

-Scott