Long piece adjustable taper jig

I am in the process of adding rigid insulation to a ceiling before putting up drywall. There was already 3-1/2" insulation that was pushed up and stapled in place leaving about a 1" air gap at the bottom above where the drywall will go. The foam has to be cut to fit between the joists. So guess what, the carpenters laid out the joist on 16 inch centers...sort of... Some of the gaps are up to 15" wide at one end and 1/2-3/4" narrower at the other end. So how do you cut a tapper. I have a long aluminum straight edge I bought years ago for using with a skill saw to break down sheets pf plywood. I first rip off a piece of the foam sheet to the maximum width needed. Then I was taping some shims to the straight edge to angle the cut piece to the needed widths at both ends. The tape idea was time consuming and not very sturdy.

I knew I needed an adjustable shim. So after a bit of thought and a few pieces of scrap wood and a short piece of T-track Here is the adjustable shim I made.
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It took about 15 minutes to make (mostly for glue drying), can be adjusted from zero to a little over 2" of taper and it has significantly speeded up the job.