A good flat-sided device is the Itaste VV V3. I have a friend who got one as her first variable voltage device and was very happy with it. It has a voltage range of 3.3v to 5v (slightly better than a Spinner).
The only downside is that when using it without the passthrough it only has 800mAh of charge, which in "vape time" means it will run roughly 6 hours before needing to be charged (that's a guesstimation it depends on your vaping style), so it might not be the best choice if you plan to go out for a long time), but you have the eGo for that.
This is about an average price (I've seen ones about $5.00 less. but they are OOS).
Innokin iTaste VV V3.0 Battery
If a little more is in your budget ($52.99) the Innokin iTaste VV4-M Express Kit 1000mAh has some upgraded features:
It still can be used as a passthrough
In addition to variable voltage, it has variable wattage, which can be easier for beginners to use.
It has more voltage range (3.0v to 6.0v), meaning you can fine turn your vape even more.
It has 1000mAh, which will give you more run time, although slightly less than your eGo - that's because using variable power uses up more power. You probably won't notice the difference.
It can fire down to 0.8Ω, meaning it will work well if you decide to learn rebuilding/
It's available in a chromatic finish (I think it's cool, you may not

)
Innokin iTaste VV4-M Express Kit 1000mAh, 15w, 0.8ohm
Overall, I think the V4 is a better choice. I didn't suggest it first because I didn't know it was out. However, if the V4 isn't in your budget a V3 would be fine. As I said, my friend loves hers.
No matter which version you choose a wall adaptor is very good add on. It will give you the freedom to use the passthrough with an electrical outlet and not be stuck at your computer.
Innokin iTaste Wall to USB - 1000mA
I also recommend another charging cable. The ones that come in the kits don't look very long. This one will work with either version of the battery device.
Micro USB Cable - Innokin iTaste VV, CLK, MVP
With my recommendations the kit comes to:
$52.99 - Innokin Itaste VV4-m 1000mAh (I might be able to find it for a little less, but it just came out and not many vendors have it.)
$6.68 wall adaptor
$1.99 Longer USB cable
Total: $61.66, plus shipping and any clearos you want to add.
I realize it looks like a lot of money, but with its features you may never need to buy another device. It's much better than what I had in the beginning, which I happily used for 6 months until I got into bigger mods. I got into mods because I became a hobbyist, not because my vape was unsatisfying.
There is another good device for roughly $15.00 less: the Innokin iTaste MVP20W. It has more power options and higher mAh, but it is box shaped and some people don't like that. It also doesn't have the passthrough feature.
ETA: Both of the Itaste VV models have 510 and eGo threading, which means you can use any clearo/tank/carto you wish.