For reference, the entire NBRSA rear sandbag rule:
2.11.2.The rear sandbag. The rear sandbag shall be a bag or combination of bags containing sand only. A vertical spacer under the rear bag will be allowed as long as it incorporates no adjustments for windage or elevation. The vertical spacer shall not contain any protrusions which can be inserted into the bench top or the sandbag. The Dunrud type rear ring spacer for use under the rear bag is legal. The rear bag shall not be contained in any manner. No metallic materials may be used in the construction of the sandbag. Tape on sandbag is legal.
IMO the Dunrud spacer's approval did not change the rule about containing the rear bag, it was an exception for a specific piece of equipment by a specific manufacturer. The Otto ring and similar products get buy because they meet the rules for a sand bag, and multiple bags are allowed. IMO a ring of any construction, attached to a metal plate that would limit the movement of the bag would in effect contain it. On the other hand if it (or they) were between spacers or under a legal intermediate piece that contacted the bag, or contacting the bench, as long as it did not protrude into it, that would not be a problem.
Having observed that Dunrud spacers introduce bounce, and that the Otto ring and Edgewood Dead Bottom do not, I can think of no reason to use the Dunrud. I traded mine off.
Hint: Supporting sandbags shapes or number of chambers are not specified, only the limits of what they may contain and be made of. Better hint: There is no rule about spacers being attached to each other. IMO they could bolt together, or interlock. as long as the contact with the bench and top sandbag is correct.