Hi Rick,
An inexpensive alternative to a tape measure (or your custom measuring device) for exact measurement of inside dimensions is the “EZ Measure Stick” available online from ezwoodshop.com. This is a fantastic little device featuring an expandable, telescoping rod assembly that, as the manufacturer says, allows you to do “No-Math Measuring” of distances especially inside measurements.
Weblink - EZ Measure Stick from EZwoodshop.com
I purchased the “Jumbo Measure Stick” package several years ago and have used it many times for household repairs and woodworking projects (current price usd 29.95) – also good for rough measurements of metal projects. This pack includes a “Mini Measure Stick” extending from 7.6 cm to 21.6 cm (3” to 8.5”) & two “Large Measure Sticks”. One large Measure Stick alone can measure from 17 cm to 116 cm (6-3/4” to 45-3/4”) & two large Measure Sticks (combined via a centre coupling) can extend from 47 cm to 234 cm (18-1/2”to 92-1/4”)
I also purchased a third Large Measure Stick which features triangular-shaped ends for corner-to-corner measurements – Alas, ezwoodshop doesn’t seem to sell these anymore.
On first sight I thought these measure sticks might not be accurate for the following reasons:
a) too flexible, or
b) would slip out of the measured distance either during removal (e.g. from a constrained space) or during transport to the destination, or
c) be unable to accurately mark down the measured distance at the destination.
Wrong on all counts - In reality, the rod assembly is just flexible enough, and the rod-to-rod fit is just stiff enough that the assembly doesn’t move. Also, each end has a “sharp, angled tip” which lies flat and allows marking with either a pencil or a scriber.
Gary Kingsep