Simple buy a bar clamp from Harbor Freight $17, cut a 10" Sq. piece of cardboard Place it on the outside of the rim and push the clamp up against the wheel and then measure from the other side and make sure it goes all the way up to the cardboard. This way you’ll never need anyone to help you again. Below is a link to the clamp… https://www.harborfreight.com/hand-tools/clamps-vises/heavy-duty-ratchet-bar-clamp-spreader-95027.html
I mark my axle keys every 5mm with a dremmel and know what the shortest mark represents so it’s easy to adjust and go from there.
When I am by myself I will push the tape measure through the axle and tape it to the edge of the rim with Gorilla tape and measure to the other lip. Maybe slighly off but it tells me what I need to know.
What Bill Holt said above. A tape measure thru the axle and in my case a small length of aluminum angle to hook the tape on. After a few tries I found I could do it without help. Virtually free.
what if you use different hubs?
Also if you change rims…
I use the measure from bearing face to hub method for race day measurements. If we are pushing 55-1/8” then we check max width with a tape inside the axle after moving wider. The rest of the time just relative moves using the adjustable T from this cheapo Harbor Freight kit.