Leaving aside the subject of building hours for the moment...what do you hope to accomplish with a flight engineer certificate? Are you hoping to "build hours" toward a career as a professional flight engineer (PFE)? Experience as a FE is not pilot experience. SIC time is, but SIC or FE time is only really of any value if it leads to employment as a PIC.
Today, few FE positions exist. Of the few that exist, almost all are filled by PFE's; these are not upgradable or upgrading positions. You start as a FE, you finish as a FE. You don't move from seat to seat. A few firms still do this, but most firms using FE's hire professional flight engineers who stay in that position.
Your mechanic certificate is a plus; it's something desirable in a PFE. However, if you're hoping to gain experience toward a pilot position, you may be barking up the wrong tree. You're far better off gaining more pilot experience, rather than trying to get your foot in the door by seeking employment as a FE.
Back to the subject of building hours, don't do that. Build experience. Building hours is wasteful. Falsify your logbook; write the time in your logbook if that's all you're after. That's all hours are worth. Instead, build experience. You can fly one hour of time, or one hour of experience, but there's no comparison between the two. I meet a lot of pilots with 15,000 hours, but much of it is nothing more than hours...not really valueable experience. Sitting and monitoring.
For you, at a time early in your career when you want to solidify and form good habits and build a base of experience upon which to grow some judgement...building hours is counter productive and wasteful. If you go rent an airplane and fly for an hour, you can spend an hour droning about, or you can concentrate on slow flight, approaches, stalls, landings, etc...you can come away with an hour of experience. The difference is entirely up to you.
Build experience, not hours.