I don't believe you need a riser for an Eotech. As I understand it it was designed to work with the AR-15's sight plane which is ~2.5" over the bore. The Eotechs, as I understand it, as this high already without riser.

Shorter dots like the Aimpoint Micro, TRS-25, Primary Arms Micro, Aimpoint Comp series and the like are all less than that and to get them in the right height to use them as the AR-15 was designed require a riser.

Just remember there is nothing magical about a "co-witness" all it means is you can see the sights through the other sight. It does not help with your shooting. If it is an absolute co witness it may speed up the transition from a broken red dot sight to your iron sights, but that's only marginal. The absolute co witness also clutters your field of view.