If you do a google search for self made or modified tenkara rods.
For example this search phrase - テンカラ竿を自作し - To make your own tenkara rod. Among some of them will be rods where the line runs inside the rod blank, with some mechanism to store the excess line.
Mostly self made or modified rod activities are found on Japanese tenkara websites. Because - Not many tenkara anglers in the west thus far are posting about their projects or even attempting them. Dr Tom is one of the few I recall seeing. That was more than just modifying the shape of the grip or changing it to a different material.
Several projects will show up where they modify a tenkara rod so that the line runs through the middle, or inside of the rod, so that the line exits the rod tip. Others take a rod manufactured as an inner line or line through the middle rod, and modify it for tenkara fishing. These rods are made and sold to use a reel of some kind. Mostly they seem to be salt water rods.
It is an idea that keeps attracting imaginations. For practical reasons. With a way to store extra line the idea provides a quick way to change line length. Many projects are rather crude. Others more refined. The ones where they use bulky or heavy line storage systems are unappealing to me. Others where the line storage system is small, light, and streamlined are more appealing.
As far as I am aware the only commercially made and sold “inner line tenkara” rods are the ones made by 柏木重孝（かしわぎ しげたか）、Kashiwagi Shigetaka-san.
I find him an interesting guy with unique ideas. His ideas are a bit outside, well, maybe way outside, most ideas of what “tenkara is”.
Indeed I have seen his style of tenkara , 柏木流 (Kashiwagi-ryū) called Hybrid Tenakra.
However, his background is interesting: creator of ZEAL lures, author of articles, VHS and DVDs about bass fishing, etc. His blog is also interesting.
Blog:
http://happy.ap.teacup.com/kasiwagitenkara/
I think I have discovered at least six websites selling his rods, kebari, and other products, the following links are to 2 of them:
http://starkingfishing.web.fc2.com/tenkara.html
https://kashiwagiryu.stores.jp/
I find rods where the line runs though the inside generally described with either of these phrases.
中通し, middle through. However, google does not always translate it that way.
インターライン , interline.
Doing a google search with either of these 2 phrases will find several examples of both the refined and the crude attempts at making an inner line tenkara rod.
中通し・テンカラ竿・を自作し… Through the middle ・tenkara pole ・self made.
インターライン・テンカラ竿・を自作し… To make your own interline Tenkara pole.
The first one finds more results.
Generally I find the ones made using a Fly Rod Reel or a Round line spool of some kind unappealing. The ones that use a moderately long & narrow line winding mechanism of some kind more appealing. Either self made one or using something similar to the clip on Owner line winding shikakemaki – such as the BS-25 or BS-10 models or similar product from other companies.
『オーナーＢＳ‐２５ベスト仕掛巻』or BS-10 ベスト仕掛巻 .
An Interesting Essay about this idea:
The link is to the Google translation in to English. It’s fairly good. But a couple of notes about the translation is below the link. Ah, you might be glad later, if you read it first. 
フライフィッシングとテンカラ釣りの融合
Fusion of fly fishing and tenkara fishing.
Fusion of Fly Fishing and Tenkara Fishing
First the easy bit.
Bristles or Hair Burr, should be Kebari. Ten Color = Tenkara, Hera Fishing, a type of Carp Fishing.
The ah, problem troubling bit. 
Google has a real problem with this phrase, 木化け・石化けは. in the bulk translation of the whole website. Found in one of the last chapters. And makes one of those unfortunate R-rated translations into English. (if you take a look at the romaji you can probably figure out why) Google translates 木化け・石化けは into Trees and Fossil F__, well, it rhymes with bad luck. 'Nuff said. It should translate as something like Garbled Trees and Garbled Stones.
Here is a good picture of 木化け・石化け, which should give you a good idea of the point the author was trying to make.
http://photozou.jp/photo/show/571102/44330954
A Different approach to making a through the middle rod for Tenkara on the amagochan6332 YouTube channel.
He does not title this video as Tenkara, but of the 28 or so videos he has uploaded, most are labelled as Tenkara videos. {including some with Dr. Ishigaki from a year ago and from 4 months ago fishing in Hokkaido]
アメマス, Amemasu , aka Salvelinus Leucomaenis 61cm (24 inches)
What amagochan3662 has done is modify a 5 meter Uzaki Nissin Pro Spec black sea bream through the middle rod, NISSINの黒鯛用中通し竿を, by covering the reel seat with a Cork Grip, and I think, added an Owner type line winder, below the cork grip. A nice view of it is near the end of the video.
http://www.u-nissin.co.jp/item/409
There are a few more videos uploaded at about the same time, a month ago, fishing with the same rod.
I expect - Opinions will vary about whether this Is Tenkara or Is Not Tenkara.
To OK maybe it is 10 Colors Tenkara, just not a color I want to color with.
I think Amagochan6332’s effort was fairly refined.
Q: 10 Colors Tenkara or to far out that the Tenkara spirit is lost??
Why I find words interesting: （改悪）= Worsened. But then, 改・悪 Translates as Improvement / evil. Maybe an accurate translation, maybe not. But amusing. 
