For starters, review the SRM and AMM, If you're lucky there may be a standard repair that covers your situation or is similar and can be adapted.
Also take a look in ac43.13 for guidance on the design of the repair.
For substantiation - in absence of verifiable internal loads data, you would want to demonstrate equivalent strength to the baseline structure. I.e. analyse the affected details to demonstrate that the doublers and joints that you have added restore the capability ( ultimate strength, and stability) that was removed from the baseline structure. Be conscious of not introducing poor fatigue details in the process.
Is there a senior engineer who can check your work?

I would absolutely not rely on a standard SRM repair or other basic repair guidance for this. You are not repairing damage, and you cannot simply take advantage of allowable damage to substantiate the hole enlargement because you are doing this as part of a non-OEM installation. Please also do not just show equivalent strength with baseline structure and call it good, that is probably not sufficient.

There are so many considerations... where to start.

Are you adding any load to these frames? Where are these avionics boxes installed?

You need to substantiate the net area loss in the frame. If you have non-OEM loads, you can't simply compare to adjacent minimum cross sections.

The bottom line is, you need to show the frame material can handle the maximum load it is subject to. So you either need to know the value OEM load + modification induced load, or you need to show the OEM margin is sufficient to cover load increase, or you need to show the amount of reinforcement you install is commensurate with the load increase.

You are talking about harnesses which are normally very low mass, but I'm concerned about what else might be done in this installation.

If you are starting by posing a question in this forum, you are starting in the wrong place to be honest.

"Aerobatic general aviation aircraft"... do you know which CFRs you need to comply with? Is this an STC? Who is approving this?

Keep em' Flying
//Fight Corrosion!

Yeah... get a DER on the matter.
Once upon a time there was a shop in town that installed new antennas in a composite plane GA plane. They had no idea the skin was a fiberglass foam sandwich. torqued the hardware down, crushed the sandwich. I heard a few were totaled after they couldn't decide on a repair.

Agree with the two people posting above. You need an experienced structural analyst/DER.

"OEM margin is sufficient to cover load increase"... or in absence of oem data, which nobody outside the OEM will have access to, design and substantiate a repair assuming baseline ms=0. I.e. restore the structural capability you are removing.
In any case seek advice and support from someone who knows what they're doing and has the appropriate delegations/authorisations. You won't get an answer on eng tips.

Like I said, it is not necessarily enough the restore the structural capability you are removing - if you are modifying the airplane such that it may have additional load over OEM. If you only return the structure to the OEM design condition.

We don't know all the details. You can't necessarily treat it as a repair. It should be treated as a modification. The distinction:

A repair restores the structure to TCDS basis.
A modification may show the structure acceptable for alternate or additional load.

We really should not be giving specifics on how to substantiate it since it is largely speculation.

Keep em' Flying
//Fight Corrosion!

Fair enough. My *assumption* is that he/she is routing a wiring harness in a fairly restricted space. Being an aerobatic aircraft it's hardly likely to be much more than that. But you're right, we don't know.

concur with above ... get professional help ! someone to help with your STC (design, analysis, certification).

another day in paradise, or is paradise one day closer ?