Hi Jiggy,
The custom transom mod works great, and I say that because I haven't been motivated to conceive a new version. It holds the engine, sonar and now three rods. I basically attached one of those expandable wall clips people use for hanging garden tools against the wall. This allows me to cram in a third smaller 7ft rod between the two that sit outwards from the boat. It lays across the top of both extender arms closer to the water and allows me to run smaller lures for smaller fish species.
I'll post some pics next time I'm out.
I use the Humminbird 120 Fishing Buddy fishfinder. It's basic but it works well and because it's all in one it's easy to clamp into the transom. Here's a link.
http://www.humminbird.com/Freshwater/Product-Category/Fishin-Buddy/
As for downriggers, they're not something I've had to deal with as my target fish - Pike - tend to hang out in the shallows. This means everything about my setup is designed to allow me to troll in waters as shallow as 3 feet and not get snagged up. If I ever decide to target Ferox Trout I'll worry about downriggers.
Having I said that, if you really need to go deep, I'm pretty sure you can use a combination of planers and paravanes to draw your lures both out from the boat and down without the need for an expensive and cumbersome onboard downrigger. See these links.
http://www.amazon.co.uk/Orange-Deep...1bZ-7X5tyL&dpSrc=sims&preST=_AC_UL70_SR70,45_
http://www.amazon.co.uk/Redesigned-...e=UTF8&qid=1445023736&sr=1-10&keywords=planer
I have never used them myself, but it may be a half way solution for your needs that prevents you having to make onboard modifications. I know other anglers who fish deeper lochs and they use them all the time.
If I were tasked to install onboard downrigger however, I'd design something that sits in the middle of the boat and stretches back top where you're sitting. Probably a hinged setup where it's placed down over the oar holders on both sides and lays flush with the boat to attach your downriggers.
Sort of shaped like a big H, with the centre line of the H running across the middle of the boat. This would give you a little bench in front of your seat to hang lures and fit cup holders. Also, the top ends of the H could be fitted with Scotty rod holders for when you have company. The whole thing could be easily secured to the oar holders on both sides. Make sense?
Actually I just took half an hour away from writing this to work up a graphic, hope this helps.