purigen? is that like chemi-pure?
i would try to talk you out of using that if you were asking advice on it.
so, i would put as large of a sump/ refugium under that tank as i could. you have plenty hanging on the back of that tank as it is. :wink: ( i have a HO skimmer as well)
your 29g will work. if it is glass, all you need to do to it is simply have somebody cut you a piece of acrylic (or DIY, it will be a snap) that will dam one end from the other. cut some kind of strainer section in the top area and
glue it in the tank. leave enough room for a submersible pump in one end and you're done. buy yourself a pump and if you really don't wanna pipe anything, buy some hose and barb fittings.
you'll need a valve and some type of overflow (unless you got the predrilled 72g? in which the predrilled will simplify it even more.
more good news, you will be able to place your heater down there.
or you can blow another hundred on what i view as an inferior method.