Buy ~12 ft 2" webbing. It should cost you about $12.
Double it over and about 1 ft from the fold, loop it to tie a simple
knot. This is your step.
Tie the bitter ends to the padeye in the floor of the RIB. The length can be adjusted for people of different heights.
To board, as you hold onto the RIB handles, with your foot in the bottom of the strap in the
water, in one motion, STEP on the bottom of the strap and push your leg outboard.
The effect is to automatically launch your chest onto the top of the pontoon and allow you to swing your other leg over.
This solution is inexpensive, weighs little, is easy to carry in your duffle, and is adjustable for different sizes of people/RIBs. You may want to buy >12 ft
webbing depending on the size of your RIB.
I recently bought a 25 ft 2" yellow towing strap from Home Depot for under $9, but I have not tried this material as a boarding strap. The material is a little heavier than the webbing I used for the boarding strap in this video.