Reddit mentions of CyclingDeal Conversion Kit Fixie Bike Single Speed Compatible with Shimano Sram Cassette Freewheel Hub Adaptor - for Mountain and Road Bike Gear Cog Spacers 16 Teeth

The picture is of a freehub, not a cassette.

You can easily make that a single speed, but not a fixed gear.

Simply get a single cog and spacer kit designed for cassette wheels.

http://www.amazon.com/Conversion-Fixie-Single-Shimano-Adaptor/dp/B006WRW45O/ref=sr_1_3?s=sporting-goods&ie=UTF8&qid=1413139008&sr=1-3&keywords=single+speed+conversion+kit

You will need to use a chain tensioner, use your existing rear derailleur as a chain tensioner, or have horizontal-enough dropouts to be able to move your rear axle enough to tension your (shortened) chain.

I guess that would work, but the only problem is eventually you'll be running a nasty chainline. I only paid $15 for the cog and spacers plus I have some replacement cogs (from old cassettes) for when stuff wears out. Otherwise I'll happily pay the $15 again.

Here's: the cog/spacer kit I got: https://www.amazon.com/dp/B006WRW45O/ref=cm_sw_r_sms_apip_gEjTj3QBqwcC3

I am about to do this, and was considering this little spacer and cog kit.

I'm curious if anyone can endorse this as a decent option, suggest a different kit, or suggest buying spacers and a cog separately.

I haven't seen something quite like this and am open to ideas.

I'm actually doing the same thing. I just built a new road bike and have some parts left so I will be building a budget single speed. I have a carbon frame, mavic wheels with 8,9,10 gear hub (will add this http://amzn.to/1M078HZ). I will also be getting this crankset http://amzn.to/1UCmyp1, this cheap brake setup http://amzn.to/1MSSi0I, some chain and a shorter stem since the frame is pretty large. Most likely will paint everything black with a gold chain. Will be super light since the frameset is carbon.

So now I feel like a jerk because I know they USED to, but I can't find it. Maybe they stopped making it. Whatever.
https://www.amazon.com/dp/B006WRW45O/ref=cm_sw_r_cp_apa_guEJBb3S7NVTP

This should work for you; or something similar. I like it because you can align your sprocket and chainring so there's no rub or noise from lateral play. Will take some finesse to get completely silent.

i had a road bike for 5 years, i still do but its in the garage last 4 weeks, because i bought fixed gear and i ride it everyday. dont have a car or brakes on my bike, and i love it. no freewheel, one less bearing. or you can buy this thing from amazon https://www.amazon.com/Cyclingdeal-Conversion-Shimano-Adaptor-CyclingDeal/dp/B006WRW45O/ref=sr_1_2?crid=1EC3K206TG2I7&keywords=single+speed+conversion+kit+bike&qid=1563848343&s=gateway&sprefix=single+speed+conv%2Caps%2C160&sr=8-2 and take your front derailleur and one of chain rings off

To be honest, I'm not really sure what a chain tensioner is. I just know the guy in the video was referring to one.

The conversion kit I saw was this: http://www.amazon.com/Conversion-Fixie-Single-Shimano-Adaptor/dp/B006WRW45O/ref=sr_1_1?ie=UTF8&qid=1405727927&sr=8-1&keywords=flip+flop+hub

...which I'm only now realizing does not convert in the direction that I'm talking about or want.

The frame set is an older Fuji. I tried to track down the year, but couldn't get the model/serial to line up, so I think it's anywhere between mid to late 90s to mid 2000s (it has the older logo).

It has the fork, and I THINK a headset... though I'm not sure. Is that what goes on top of the handlebars, or under them?

As far as I know you can do this 2 ways: get a new rear hub which is a single speed or a flip flop hub which has one side for single gear use and one side for fixed gear use, or you can one of those single gear cogs that you mount on to an existing cassette hub it's a converter sort of thing.

Now, there's no need to get a completely new rear wheel, but if you're re-using the rim, you'll have to take it apart to mount a new hub. It's simpler to just get a rim with a single gear hub already installed. Or get the previously mentioned adapter for your cassette.

Overall, I would say it's easier (not cheaper) to get a wheel that's made for single speed use. Also, keep in mind the spacing of your rear-drop outs as different speeds often have different sizes of dropout.

If you have horizontal dropouts, it's actually pretty easy. Basically the issue is taking up slack in the chain. There exists some products you can use if you have standard dropouts. An eccentric bottom bracket would be your best bet. Problem Solvers makes a chain tensioner you can put on your dérailleur hanger if you can't do the eccentric be.

Eccentric bottom bracket - this one is for bb30, I think you can get one for square taper style, too.. Phil wood makes one. Ask your lbs.

Chain Tensioner

You need to replace the cassette with a single sprocket.

You'd need to remove your derailleurs.

You would need to either replace your crank to a suitable one, or you can just take off all but one of the chain rings without too much issue.

Remember to support your local bike shop. They can get set up single speed much easier than on your own.

edit: yeah, i read it wrong. yeah it's not easy.