Well...
SS bikes that'll see regular use and/or any real mileage really do need some way to keep the chain tensioned. Unless you like frequent roadside and workshop fiddling.
Particularly considering that on a derailer gear bike, the chain, the sprockets and the chainrings are DESIGNED to let the chain come off easily.
And since replacing a rear derailer with a chain tensioner doesn't make much sense,(low-end derailers are often cheaper than dedicated chain tensioners), I wouldn't recommend it at all unless the bike has (semi)horizontal dropouts.
Even if it has, you're still using a systemd designed to derail for the purpose of not derailing.
You may end up wanting to keep the front derailer as a chain guide/chain catcher.
And you write:" I plan on keeping the chain as straight as possible."
That's a good start. Have you checked that you can find a nice, useful gear at that alignment?
And you write:".. and giving it as much tension as i can. "
No, that's NOT how you do it.
Chainrings and sprockets usually aren't exactly centered. Meaning you'll have tight spots and slack spots.
If you set the chain tight at the slack spot, you'll get a wicked amount of chain tension at the tight spot.
It'll cause rapid wear on chain, sprocket, chainrings and bearings and can also pull your rear wheel askew.
The chain tension should be so that you can lift the chain 1/2" mid-run at the tightest.
Any more is no help to you.
Your best option is to buy a new derailer. Low-end ones can be had cheap.
A bike shop might let you trade a used one for a pack of donuts.
Or just buy one:http://www.amazon.com/Dcolor-Racing-Bicycle-Silver-Derailleur/dp/B012CM3T92/ref=sr_1_155?ie=UTF8&qid=1453196199&sr=8-155&keywords=rear+derailleur
If you're that unconcerned about performance that you'd be OK with a DIY SS, this one should work for you.
How much tinkering are you willing to spend on avoiding spending $6.11?