Yes, I think a flat bar across and possibly 2 vertical pieces on each side. I don't think it has to be over-engineered or anything. I have booster seats like AZG23 and those headrests are pretty flimsy, yet we trust them
As for the guides, if I can't get them with the headrest, I think some tubes with nylon bushings of some sort would work.
EDIT: I've even thought about just hard-mounting the headrests without guides. The drawbacks would be possibly no fold and tumble. Also, it would have to be done with the headrest guides already through the cover - otherwise you wouldn't be able to get the cover back on.
EDIT 2: Unless maybe you had a zipper installed in the top of the seat cover???