When I needed a different length dipstick and couldn't find one I went to my local metal supplier and spent $3 buying a 6'x3/16" rod of 6061 aluminum scrap. You press the retaining pin out of the old dipstick with a drill bit or punch and then put the new rod in. Undersized drill for the pin while it's in the cap, then pull it out and do a final sized drill pass so you don't enlarge the hole in the cap. Luckily I had a friend with the same engine in the hangar and I was able to just copy his dipstick length and markings, however you can manually calibrate them by adding one quart of oil at a time, and marking the level lines with a file. I even stamped quart number markings with a cheap set of harbor freight stamps that I had laying around for stamping data plates.
For $3 I had enough material to make several dipsticks this way. I'd give you the leftovers but I already gave it away to another builder. If you don't have an existing dipstick to get the cap from, you can often find them cheap at salvage yards or random assorted lengths off ebay. The only other option I could find at the time was brand new from Lycoming for around $300, so I saved 99% which is pretty funny.
