Got a small sandwich sized freezer ziplock under the soft corner from the converter (vent) end. Couldn't get anything in there from the bottom. The supporting styrofoam is really pretty solid everywhere except that rear corner.
Slipped a length of vinyl tubing into the bag and zipped it mostly closed. Slid the baggie under the corner, attached the tubing to the nozzle of my foam and slowly filled the bag. Stopped when it was approaching the zip of the bag. Carefully backed the tube out of the bag (with my gloves on Wavery
) and managed not to get foam on anything, including myself.
Foam continued to expand for a few minutes, creeping out of the opening in the zipper, but slowly and doesn't appear to have touched anything that it shouldn't.
I'll go back tomorrow after it has had some time to cure and test it. I imagine being contained in a plastic bag will slow the cure time down considerably. Likely won't bother trying to remove any of the excess. Let sleeping dogs lie!
Thanks to Travis for the bag idea, and Wavery for the ziplock tip.