I picked up a set of these to try out, seems to be a similar build/OEM for the topper we have. The rails look compatible.
https://openroad4wd.com/products/openroad-truck-cap-cross-bars
Finally got around to putting it together and mounted. Probably going to leave the default mount in place for the time being vs create brackets to use with the Mars adapters. I have a feeling I wonāt really care to use the tonneau much with the topper, but will try it out and decide later...
The sides donāt open at all. The only access you would have from the sides would be through the window. So if you get fully enclosed with no window, your only option is through the back.
There are ones available, like this
https://www.rivianforums.com/forum/threads/custom-made-rivian-r1t-truck-cap-topper-autofitpro-group-buy-interested.37848/
15% right now, thinking about picking up 4 of these. You get a pair per order, so just need two sets.
https://marsag.co/products/accessory-port-adapter-rivian