^^Depends how much you want to spend and how close you want to be to historical accuracy and/or current furniture/sight packages. That side mount which the C-More style RDS is on is an ICS one which cost about £50 new but, if your AK has a dovetail mount on the side of the receiver, you can get a decent enough one pretty cheap, like this or the QD version like this, or a very simple one like this. Or, depending on whether your AK is a TM compatible design, you could go for a full RAS like this, which puts a nice and solid top rail in the more usual position without the need for the dovetail on the side. There is also another type which bolt on underneath and through the rear sight retaining screw, like this, but they're pretty shitty so I wouldn't recommend one.
There are also modern Russian stylee solutions based on the Zenit company's uber tacticool furniture - LCT do a B13 clone (which is a top cover with an integrated rail) but TWI have the best rep afaik, although none of the Zenit clone gear is what you could call reasonably priced, or even necessarily value for money. It does truly look TDB though
The thing is that red dots are supposed to be used with both eyes open so you may be surprised by how useful a 25-32mm RDS on an RIS handguard can be*, but another option is to put a £25 MRDS on one of these which replaces the rear sight. You can get a dedicated mount for a Docter MRDS which replaces AK rear sights, but that's not a cheap solution either. *I've tried running a clone EOTech 551 on top of an RIS handguard on my tac AK, but my 551 has an EOLAD laser on the side which means that the whole thing is a bit of a fat chunk to have in your point of view, so it only lasted a single morning of skirmish experimentation, but I expect that a simple 551 or 556 clone would work ok...