I've started making my own heated stuff a number of years ago.
I think from the old 'Sue's homemade heated gear' info.
I found it worked quite well, but learned some things.
1. Like mentioned, keep the wire as close and snug to your body as you can. If it sits away just a bit, it won't work nearly as well.
2. The connection to the power cord needs to have very good strain relief so it doesn't get damaged when you forget to unplug.
In the past I wired up the liners of my various jackets, but got a Roadcrafter and it didn't have a liner. I started looking into jackets to wire up, but by the time I got one that had elastic to keep it snug, some sort of windproofing- I just bought one of the Warm-n-Safe ones.
A little comparison, from a cheap...er... thrifty guy.
Having the elastic on the jacket that made it very snug but comfortable is really important to how well it works. Maybe some don't get as cold as I do, but it made a huge difference.
The heated collar was very nice and well worth addition.
Wiring that allows plugging in gloves/socks - either in parallel or separately is well work it also.
In the past I ran wires and controllers separate for jacket/gloves/socks. Now I'm running one controller for all. A bit of a trade off, since optimum temperature for each item isn't always possible, but I like it better since it is one plug-in and I have a handlebar mounted heat controller.
That said, it was well worth it for me to just buy the Warm-n-Safe jacket.
For those that don't want to spend the money (or don't have it) I'd keep the points in mind when you make one. May or may not be important to you but all worthy features for me.
Gloves, I've never tried to make any, but had some Widder ones in the past. Warm, but bulky as heck and I never liked them.
Sold them and got he Warm-n-Safe 'Ultimate' heated glove. No insulation, thin like summer gloves, knuckle guards (more on that later) and a waterproof layer.
I tired them with and without the waterproof layer and couldn't tell any feel difference at all, but the ones with the layer were noticeably warmer. I guess they don't make them like that anymore and that is too bad. Others that don't have Reynolds and problems with cold fingers may not notice the difference.
On the knuckle guards, I got in a wreck about a year and a half ago (my own dumb fault) and hit my hand very hard on something (probably the car). I had a huge bruise, but I'm convinced that my hand would have been broken without the carbon knuckle guards. I'm a big believer.
I know I'm sounding like a Warm-n-Safe rep or something, but I'm just a very satisfied customer (there customer service is fantastic).
So, keep it snug, one thin layer under, wamer layers over. Don't have the wired overlap or get too close to each other (hot spots), use decent sized wire TO the heated wire.
OSSA Explorer 2014 AND 1974, Ducati Multistrada 'S' custom-Lotus M100- '68 Benelli dynamo trail- 76 OSSA GPII Phantom (project) 86 XT350